PuTTY User Manual
=================
PuTTY is a free (MIT-licensed) Win32 Telnet and SSH client. This manual
documents PuTTY, and its companion utilities PSCP, PSFTP, Plink, Pageant
and PuTTYgen.
_Note to Unix users:_ this manual currently primarily documents the Windows
versions of the PuTTY utilities. Some options are therefore mentioned
that are absent from the Unix version; the Unix version has features not
described here; and the pterm and command-line puttygen utilities are not
described at all. The only Unix-specific documentation that currently
exists is the man pages.
This manual is copyright 2001-2005 Simon Tatham. All rights reserved. You
may distribute this documentation under the MIT licence. See appendix C for
the licence text in full.
Chapter 1: Introduction to PuTTY
--------------------------------
PuTTY is a free SSH, Telnet and Rlogin client for 32-bit Windows
systems.
1.1 What are SSH, Telnet and Rlogin?
If you already know what SSH, Telnet and Rlogin are, you can safely
skip on to the next section.
SSH, Telnet and Rlogin are three ways of doing the same thing:
logging in to a multi-user computer from another computer, over a
network.
Multi-user operating systems, such as Unix and VMS, usually present
a command-line interface to the user, much like the `Command Prompt'
or `MS-DOS Prompt' in Windows. The system prints a prompt, and you
type commands which the system will obey.
Using this type of interface, there is no need for you to be sitting
at the same machine you are typing commands to. The commands,
and responses, can be sent over a network, so you can sit at one
computer and give commands to another one, or even to more than one.
SSH, Telnet and Rlogin are _network protocols_ that allow you to do
this. On the computer you sit at, you run a _client_, which makes a
network connection to the other computer (the _server_). The network
connection carries your keystrokes and commands from the client to
the server, and carries the server's responses back to you.
These protocols can also be used for other types of keyboard-based
interactive session. In particular, there are a lot of bulletin
boards, talker systems and MUDs (Multi-User Dungeons) which support
access using Telnet. There are even a few that support SSH.
You might want to use SSH, Telnet or Rlogin if:
- you have an account on a Unix or VMS system which you want to be
able to access from somewhere else
- your Internet Service Provider provides you with a login account
on a web server. (This might also be known as a _shell account_.
A _shell_ is the program that runs on the server and interprets
your commands for you.)
- you want to use a bulletin board system, talker or MUD which can
be accessed using Telnet.
You probably do _not_ want to use SSH, Telnet or Rlogin if:
- you only use Windows. Windows computers have their own ways
of networking between themselves, and unless you are doing
something fairly unusual, you will not need to use any of these
remote login protocols.
1.2 How do SSH, Telnet and Rlogin differ?
This list summarises some of the differences between SSH, Telnet and
Rlogin.
- SSH (which stands for `secure shell') is a recently designed,
high-security protocol. It uses strong cryptography to protect
your connection against eavesdropping, hijacking and other
attacks. Telnet and Rlogin are both older protocols offering
minimal security.
- SSH and Rlogin both allow you to log in to the server without
having to type a password. (Rlogin's method of doing this is
insecure, and can allow an attacker to access your account on
the server. SSH's method is much more secure, and typically
breaking the security requires the attacker to have gained
access to your actual client machine.)
- SSH allows you to connect to the server and automatically send
a command, so that the server will run that command and then
disconnect. So you can use it in automated processing.
The Internet is a hostile environment and security is everybody's
responsibility. If you are connecting across the open Internet,
then we recommend you use SSH. If the server you want to connect
to doesn't support SSH, it might be worth trying to persuade the
administrator to install it.
If your client and server are both behind the same (good) firewall,
it is more likely to be safe to use Telnet or Rlogin, but we still
recommend you use SSH.
Chapter 2: Getting started with PuTTY
-------------------------------------
This chapter gives a quick guide to the simplest types of
interactive login session using PuTTY.
2.1 Starting a session
When you start PuTTY, you will see a dialog box. This dialog box
allows you to control everything PuTTY can do. See chapter 4 for
details of all the things you can control.
You don't usually need to change most of the configuration options.
To start the simplest kind of session, all you need to do is to
enter a few basic parameters.
In the `Host Name' box, enter the Internet host name of the server
you want to connect to. You should have been told this by the
provider of your login account.
Now select a login protocol to use, from the `Protocol' buttons.
For a login session, you should select Telnet, Rlogin or SSH. See
section 1.2 for a description of the differences between the three
protocols, and advice on which one to use. The fourth protocol,
_Raw_, is not used for interactive login sessions; you would usually
use this for debugging other Internet services (see section 3.6).
When you change the selected protocol, the number in the `Port'
box will change. This is normal: it happens because the various
login services are usually provided on different network ports
by the server machine. Most servers will use the standard port
numbers, so you will not need to change the port setting. If your
server provides login services on a non-standard port, your system
administrator should have told you which one. (For example, many
MUDs run Telnet service on a port other than 23.)
Once you have filled in the `Host Name', `Protocol', and possibly
`Port' settings, you are ready to connect. Press the `Open' button
at the bottom of the dialog box, and PuTTY will begin trying to
connect you to the server.
2.2 Verifying the host key (SSH only)
If you are not using the SSH protocol, you can skip this section.
If you are using SSH to connect to a server for the first time, you
will probably see a message looking something like this:
The server's host key is not cached in the registry. You
have no guarantee that the server is the computer you
think it is.
The server's rsa2 key fingerprint is:
ssh-rsa 1024 7b:e5:6f:a7:f4:f9:81:62:5c:e3:1f:bf:8b:57:6c:5a
If you trust this host, hit Yes to add the key to
PuTTY's cache and carry on connecting.
If you want to carry on connecting just once, without
adding the key to the cache, hit No.
If you do not trust this host, hit Cancel to abandon the
connection.
This is a feature of the SSH protocol. It is designed to protect you
against a network attack known as _spoofing_: secretly redirecting
your connection to a different computer, so that you send your
password to the wrong machine. Using this technique, an attacker
would be able to learn the password that guards your login account,
and could then log in as if they were you and use the account for
their own purposes.
To prevent this attack, each server has a unique identifying code,
called a _host key_. These keys are created in a way that prevents
one server from forging another server's key. So if you connect to a
server and it sends you a different host key from the one you were
expecting, PuTTY can warn you that the server may have been switched
and that a spoofing attack might be in progress.
PuTTY records the host key for each server you connect to, in the
Windows Registry. Every time you connect to a server, it checks that
the host key presented by the server is the same host key as it was
the last time you connected. If it is not, you will see a warning,
and you will have the chance to abandon your connection before you
type any private information (such as a password) into it.
However, when you connect to a server you have not connected to
before, PuTTY has no way of telling whether the host key is the
right one or not. So it gives the warning shown above, and asks you
whether you want to trust this host key or not.
Whether or not to trust the host key is your choice. If you are
connecting within a company network, you might feel that all
the network users are on the same side and spoofing attacks are
unlikely, so you might choose to trust the key without checking
it. If you are connecting across a hostile network (such as the
Internet), you should check with your system administrator, perhaps
by telephone or in person. (Some modern servers have more than
one host key. If the system administrator sends you more than one
fingerprint, you should make sure the one PuTTY shows you is on the
list, but it doesn't matter which one it is.)
2.3 Logging in
After you have connected, and perhaps verified the server's host
key, you will be asked to log in, probably using a username and a
password. Your system administrator should have provided you with
these. Enter the username and the password, and the server should
grant you access and begin your session. If you have mistyped your
password, most servers will give you several chances to get it
right.
If you are using SSH, be careful not to type your username wrongly,
because you will not have a chance to correct it after you press
Return; many SSH servers do not permit you to make two login
attempts using different usernames. If you type your username
wrongly, you must close PuTTY and start again.
If your password is refused but you are sure you have typed it
correctly, check that Caps Lock is not enabled. Many login servers,
particularly Unix computers, treat upper case and lower case as
different when checking your password; so if Caps Lock is on, your
password will probably be refused.
2.4 After logging in
After you log in to the server, what happens next is up to the
server! Most servers will print some sort of login message and then
present a prompt, at which you can type commands which the server
will carry out. Some servers will offer you on-line help; others
might not. If you are in doubt about what to do next, consult your
system administrator.
2.5 Logging out
When you have finished your session, you should log out by typing
the server's own logout command. This might vary between servers; if
in doubt, try `logout' or `exit', or consult a manual or your system
administrator. When the server processes your logout command, the
PuTTY window should close itself automatically.
You _can_ close a PuTTY session using the Close button in the window
border, but this might confuse the server - a bit like hanging up a
telephone unexpectedly in the middle of a conversation. We recommend
you do not do this unless the server has stopped responding to you
and you cannot close the window any other way.
Chapter 3: Using PuTTY
----------------------
This chapter provides a general introduction to some more advanced
features of PuTTY. For extreme detail and reference purposes,
chapter 4 is likely to contain more information.
3.1 During your session
A lot of PuTTY's complexity and features are in the configuration
panel. Once you have worked your way through that and started
a session, things should be reasonably simple after that.
Nevertheless, there are a few more useful features available.
3.1.1 Copying and pasting text
Often in a PuTTY session you will find text on your terminal screen
which you want to type in again. Like most other terminal emulators,
PuTTY allows you to copy and paste the text rather than having to
type it again. Also, copy and paste uses the Windows clipboard, so
that you can paste (for example) URLs into a web browser, or paste
from a word processor or spreadsheet into your terminal session.
PuTTY's copy and paste works entirely with the mouse. In order to
copy text to the clipboard, you just click the left mouse button in
the terminal window, and drag to select text. When you let go of the
button, the text is _automatically_ copied to the clipboard. You
do not need to press Ctrl-C or Ctrl-Ins; in fact, if you do press
Ctrl-C, PuTTY will send a Ctrl-C character down your session to the
server where it will probably cause a process to be interrupted.
Pasting is done using the right button (or the middle mouse
button, if you have a three-button mouse and have set it up; see
section 4.11.2). (Pressing Shift-Ins, or selecting `Paste' from
the Ctrl+right-click context menu, have the same effect.) When you
click the right mouse button, PuTTY will read whatever is in the
Windows clipboard and paste it into your session, _exactly_ as if it
had been typed at the keyboard. (Therefore, be careful of pasting
formatted text into an editor that does automatic indenting; you may
find that the spaces pasted from the clipboard plus the spaces added
by the editor add up to too many spaces and ruin the formatting.
There is nothing PuTTY can do about this.)
If you double-click the left mouse button, PuTTY will select a whole
word. If you double-click, hold down the second click, and drag the
mouse, PuTTY will select a sequence of whole words. (You can adjust
precisely what PuTTY considers to be part of a word; see section
4.11.5.) If you _triple_-click, or triple-click and drag, then PuTTY
will select a whole line or sequence of lines.
If you want to select a rectangular region instead of selecting to
the end of each line, you can do this by holding down Alt when you
make your selection. (You can also configure rectangular selection
to be the default, and then holding down Alt gives the normal
behaviour instead. See section 4.11.4 for details.)
If you have a middle mouse button, then you can use it to adjust an
existing selection if you selected something slightly wrong. (If you
have configured the middle mouse button to paste, then the right
mouse button does this instead.) Click the button on the screen,
and you can pick up the nearest end of the selection and drag it to
somewhere else.
It's possible for the server to ask to handle mouse clicks in the
PuTTY window itself. If this happens, the mouse pointer will turn
into an arrow, and using the mouse to copy and paste will only work
if you hold down Shift. See section 4.6.2 and section 4.11.3 for
details of this feature and how to configure it.
3.1.2 Scrolling the screen back
PuTTY keeps track of text that has scrolled up off the top of the
terminal. So if something appears on the screen that you want to
read, but it scrolls too fast and it's gone by the time you try to
look for it, you can use the scrollbar on the right side of the
window to look back up the session history and find it again.
As well as using the scrollbar, you can also page the scrollback
up and down by pressing Shift-PgUp and Shift-PgDn. You can scroll
a line at a time using Ctrl-PgUp and Ctrl-PgDn. These are still
available if you configure the scrollbar to be invisible.
By default the last 200 lines scrolled off the top are preserved for
you to look at. You can increase (or decrease) this value using the
configuration box; see section 4.7.3.
3.1.3 The System menu
If you click the left mouse button on the icon in the top left
corner of PuTTY's terminal window, or click the right mouse button
on the title bar, you will see the standard Windows system menu
containing items like Minimise, Move, Size and Close.
PuTTY's system menu contains extra program features in addition
to the Windows standard options. These extra menu commands are
described below.
(These options are also available in a context menu brought up by
holding Ctrl and clicking with the right mouse button anywhere in
the PuTTY window.)
3.1.3.1 The PuTTY Event Log
If you choose `Event Log' from the system menu, a small window will
pop up in which PuTTY logs significant events during the connection.
Most of the events in the log will probably take place during
session startup, but a few can occur at any point in the session,
and one or two occur right at the end.
You can use the mouse to select one or more lines of the Event Log,
and hit the Copy button to copy them to the clipboard. If you are
reporting a bug, it's often useful to paste the contents of the
Event Log into your bug report.
3.1.3.2 Special commands
Depending on the protocol used for the current session, there may be
a submenu of `special commands'. These are protocol-specific tokens,
such as a `break' signal, that can be sent down a connection in
addition to normal data. Their precise effect is usually up to the
server. Currently only Telnet and SSH have special commands.
The following special commands are available in Telnet:
- Are You There
- Break
- Synch
- Erase Character
PuTTY can also be configured to send this when the Backspace key
is pressed; see section 4.16.3.
- Erase Line
- Go Ahead
- No Operation
Should have no effect.
- Abort Process
- Abort Output
- Interrupt Process
PuTTY can also be configured to send this when Ctrl-C is typed;
see section 4.16.3.
- Suspend Process
PuTTY can also be configured to send this when Ctrl-Z is typed;
see section 4.16.3.
- End Of Record
- End Of File
In an SSH connection, the following special commands are available:
- IGNORE message
Should have no effect.
- Repeat key exchange
Only available in SSH-2. Forces a repeat key exchange
immediately (and resets associated timers and counters). For
more information about repeat key exchanges, see section 4.19.2.
- Break
Only available in SSH-2, and only during a session. Optional
extension; may not be supported by server. PuTTY requests the
server's default break length.
- Signals (SIGINT, SIGTERM etc)
Only available in SSH-2, and only during a session. Sends
various POSIX signals. Not honoured by all servers.
3.1.3.3 Starting new sessions
PuTTY's system menu provides some shortcut ways to start new
sessions:
- Selecting `New Session' will start a completely new instance of
PuTTY, and bring up the configuration box as normal.
- Selecting `Duplicate Session' will start a session in a new
window with precisely the same options as your current one -
connecting to the same host using the same protocol, with all
the same terminal settings and everything.
- In an inactive window, selecting `Restart Session' will do the
same as `Duplicate Session', but in the current window.
- The `Saved Sessions' submenu gives you quick access to any sets
of stored session details you have previously saved. See section
4.1.2 for details of how to create saved sessions.
3.1.3.4 Changing your session settings
If you select `Change Settings' from the system menu, PuTTY will
display a cut-down version of its initial configuration box. This
allows you to adjust most properties of your current session. You
can change the terminal size, the font, the actions of various
keypresses, the colours, and so on.
Some of the options that are available in the main configuration box
are not shown in the cut-down Change Settings box. These are usually
options which don't make sense to change in the middle of a session
(for example, you can't switch from SSH to Telnet in mid-session).
3.1.3.5 Copy All to Clipboard
This system menu option provides a convenient way to copy the whole
contents of the terminal screen (up to the last nonempty line) and
scrollback to the clipboard in one go.
3.1.3.6 Clearing and resetting the terminal
The `Clear Scrollback' option on the system menu tells PuTTY to
discard all the lines of text that have been kept after they
scrolled off the top of the screen. This might be useful, for
example, if you displayed sensitive information and wanted to make
sure nobody could look over your shoulder and see it. (Note that
this only prevents a casual user from using the scrollbar to view
the information; the text is not guaranteed not to still be in
PuTTY's memory.)
The `Reset Terminal' option causes a full reset of the terminal
emulation. A VT-series terminal is a complex piece of software and
can easily get into a state where all the text printed becomes
unreadable. (This can happen, for example, if you accidentally
output a binary file to your terminal.) If this happens, selecting
Reset Terminal should sort it out.
3.1.3.7 Full screen mode
If you find the title bar on a maximised window to be ugly or
distracting, you can select Full Screen mode to maximise PuTTY `even
more'. When you select this, PuTTY will expand to fill the whole
screen and its borders, title bar and scrollbar will disappear. (You
can configure the scrollbar not to disappear in full-screen mode if
you want to keep it; see section 4.7.3.)
When you are in full-screen mode, you can still access the system
menu if you click the left mouse button in the _extreme_ top left
corner of the screen.
3.2 Creating a log file of your session
For some purposes you may find you want to log everything that
appears on your screen. You can do this using the `Logging' panel in
the configuration box.
To begin a session log, select `Change Settings' from the system
menu and go to the Logging panel. Enter a log file name, and select
a logging mode. (You can log all session output including the
terminal control sequences, or you can just log the printable text.
It depends what you want the log for.) Click `Apply' and your log
will be started. Later on, you can go back to the Logging panel and
select `Logging turned off completely' to stop logging; then PuTTY
will close the log file and you can safely read it.
See section 4.2 for more details and options.
3.3 Altering your character set configuration
If you find that special characters (accented characters, for
example, or line-drawing characters) are not being displayed
correctly in your PuTTY session, it may be that PuTTY is
interpreting the characters sent by the server according to the
wrong _character set_. There are a lot of different character sets
available, so it's entirely possible for this to happen.
If you click `Change Settings' and look at the `Translation' panel,
you should see a large number of character sets which you can
select, and other related options. Now all you need is to find out
which of them you want! (See section 4.10 for more information.)
3.4 Using X11 forwarding in SSH
The SSH protocol has the ability to securely forward X Window System
applications over your encrypted SSH connection, so that you can run
an application on the SSH server machine and have it put its windows
up on your local machine without sending any X network traffic in
the clear.
In order to use this feature, you will need an X display server
for your Windows machine, such as Cygwin/X, X-Win32, or Exceed.
This will probably install itself as display number 0 on your local
machine; if it doesn't, the manual for the X server should tell you
what it does do.
You should then tick the `Enable X11 forwarding' box in the Tunnels
panel (see section 4.21) before starting your SSH session. The `X
display location' box is blank by default, which means that PuTTY
will try to use a sensible default such as `:0', which is the usual
display location where your X server will be installed. If that
needs changing, then change it.
Now you should be able to log in to the SSH server as normal. To
check that X forwarding has been successfully negotiated during
connection startup, you can check the PuTTY Event Log (see section
3.1.3.1). It should say something like this:
2001-12-05 17:22:01 Requesting X11 forwarding
2001-12-05 17:22:02 X11 forwarding enabled
If the remote system is Unix or Unix-like, you should also be able
to see that the `DISPLAY' environment variable has been set to point
at display 10 or above on the SSH server machine itself:
fred@unixbox:~$ echo $DISPLAY
unixbox:10.0
If this works, you should then be able to run X applications in the
remote session and have them display their windows on your PC.
Note that if your PC X server requires authentication to connect,
then PuTTY cannot currently support it. If this is a problem for
you, you should mail the PuTTY authors and give details (see
appendix B).
For more options relating to X11 forwarding, see section 4.21.
3.5 Using port forwarding in SSH
The SSH protocol has the ability to forward arbitrary network
connections over your encrypted SSH connection, to avoid the
network traffic being sent in clear. For example, you could use
this to connect from your home computer to a POP-3 server on a
remote machine without your POP-3 password being visible to network
sniffers.
In order to use port forwarding to connect from your local machine
to a port on a remote server, you need to:
- Choose a port number on your local machine where PuTTY should
listen for incoming connections. There are likely to be plenty
of unused port numbers above 3000. (You can also use a local
loopback address here; see below for more details.)
- Now, before you start your SSH connection, go to the Tunnels
panel (see section 4.22). Make sure the `Local' radio button
is set. Enter the local port number into the `Source port'
box. Enter the destination host name and port number into
the `Destination' box, separated by a colon (for example,
`popserver.example.com:110' to connect to a POP-3 server).
- Now click the `Add' button. The details of your port forwarding
should appear in the list box.
Now start your session and log in. (Port forwarding will not be
enabled until after you have logged in; otherwise it would be easy
to perform completely anonymous network attacks, and gain access to
anyone's virtual private network.) To check that PuTTY has set up
the port forwarding correctly, you can look at the PuTTY Event Log
(see section 3.1.3.1). It should say something like this:
2001-12-05 17:22:10 Local port 3110 forwarding to
popserver.example.com:110
Now if you connect to the source port number on your local PC, you
should find that it answers you exactly as if it were the service
running on the destination machine. So in this example, you could
then configure an e-mail client to use `localhost:3110' as a POP-
3 server instead of `popserver.example.com:110'. (Of course, the
forwarding will stop happening when your PuTTY session closes down.)
You can also forward ports in the other direction: arrange for a
particular port number on the _server_ machine to be forwarded back
to your PC as a connection to a service on your PC or near it. To do
this, just select the `Remote' radio button instead of the `Local'
one. The `Source port' box will now specify a port number on the
_server_ (note that most servers will not allow you to use port
numbers under 1024 for this purpose).
An alternative way to forward local connections to remote hosts is
to use dynamic SOCKS proxying. For this, you will need to select
the `Dynamic' radio button instead of `Local', and then you should
not enter anything into the `Destination' box (it will be ignored).
This will cause PuTTY to listen on the port you have specified, and
provide a SOCKS proxy service to any programs which connect to that
port. So, in particular, you can forward other PuTTY connections
through it by setting up the Proxy control panel (see section 4.15
for details).
The source port for a forwarded connection usually does not accept
connections from any machine except the SSH client or server machine
itself (for local and remote forwardings respectively). There are
controls in the Tunnels panel to change this:
- The `Local ports accept connections from other hosts' option
allows you to set up local-to-remote port forwardings (including
dynamic port forwardings) in such a way that machines other than
your client PC can connect to the forwarded port.
- The `Remote ports do the same' option does the same thing for
remote-to-local port forwardings (so that machines other than
the SSH server machine can connect to the forwarded port.) Note
that this feature is only available in the SSH-2 protocol, and
not all SSH-2 servers honour it (in OpenSSH, for example, it's
usually disabled by default).
You can also specify an IP address to listen on. Typically a
Windows machine can be asked to listen on any single IP address
in the 127.*.*.* range, and all of these are loopback addresses
available only to the local machine. So if you forward (for example)
`127.0.0.5:79' to a remote machine's finger port, then you should be
able to run commands such as `finger fred@127.0.0.5'. This can be
useful if the program connecting to the forwarded port doesn't allow
you to change the port number it uses. This feature is available
for local-to-remote forwarded ports; SSH-1 is unable to support it
for remote-to-local ports, while SSH-2 can support it in theory but
servers will not necessarily cooperate.
(Note that if you're using Windows XP Service Pack 2, you may need
to obtain a fix from Microsoft in order to use addresses like
127.0.0.5 - see question A.7.20.)
3.6 Making raw TCP connections
A lot of Internet protocols are composed of commands and responses
in plain text. For example, SMTP (the protocol used to transfer e-
mail), NNTP (the protocol used to transfer Usenet news), and HTTP
(the protocol used to serve Web pages) all consist of commands in
readable plain text.
Sometimes it can be useful to connect directly to one of these
services and speak the protocol `by hand', by typing protocol
commands and watching the responses. On Unix machines, you can do
this using the system's `telnet' command to connect to the right
port number. For example, `telnet mailserver.example.com 25' might
enable you to talk directly to the SMTP service running on a mail
server.
Although the Unix `telnet' program provides this functionality, the
protocol being used is not really Telnet. Really there is no actual
protocol at all; the bytes sent down the connection are exactly the
ones you type, and the bytes shown on the screen are exactly the
ones sent by the server. Unix `telnet' will attempt to detect or
guess whether the service it is talking to is a real Telnet service
or not; PuTTY prefers to be told for certain.
In order to make a debugging connection to a service of this
type, you simply select the fourth protocol name, `Raw', from the
`Protocol' buttons in the `Session' configuration panel. (See
section 4.1.1.) You can then enter a host name and a port number,
and make the connection.
3.7 The PuTTY command line
PuTTY can be made to do various things without user intervention
by supplying command-line arguments (e.g., from a command prompt
window, or a Windows shortcut).
3.7.1 Starting a session from the command line
These options allow you to bypass the configuration window and
launch straight into a session.
To start a connection to a server called `host':
putty.exe [-ssh | -telnet | -rlogin | -raw] [user@]host
If this syntax is used, settings are taken from the Default Settings
(see section 4.1.2); `user' overrides these settings if supplied.
Also, you can specify a protocol, which will override the default
protocol (see section 3.7.3.2).
For telnet sessions, the following alternative syntax is supported
(this makes PuTTY suitable for use as a URL handler for telnet URLs
in web browsers):
putty.exe telnet://host[:port]/
In order to start an existing saved session called `sessionname',
use the `-load' option (described in section 3.7.3.1).
putty.exe -load "session name"
3.7.2 `-cleanup'
If invoked with the `-cleanup' option, rather than running as
normal, PuTTY will remove its registry entries and random seed file
from the local machine (after confirming with the user).
Note that on multi-user systems, `-cleanup' only removes registry
entries and files associated with the currently logged-in user.
3.7.3 Standard command-line options
PuTTY and its associated tools support a range of command-line
options, most of which are consistent across all the tools. This
section lists the available options in all tools. Options which are
specific to a particular tool are covered in the chapter about that
tool.
3.7.3.1 `-load': load a saved session
The `-load' option causes PuTTY to load configuration details out
of a saved session. If these details include a host name, then this
option is all you need to make PuTTY start a session.
You need double quotes around the session name if it contains
spaces.
If you want to create a Windows shortcut to start a PuTTY saved
session, this is the option you should use: your shortcut should
call something like
d:\path\to\putty.exe -load "my session"
(Note that PuTTY itself supports an alternative form of this option,
for backwards compatibility. If you execute `putty @sessionname' it
will have the same effect as `putty -load "sessionname"'. With the
`@' form, no double quotes are required, and the `@' sign must be
the very first thing on the command line. This form of the option is
deprecated.)
3.7.3.2 Selecting a protocol: `-ssh', `-telnet', `-rlogin', `-raw'
To choose which protocol you want to connect with, you can use one
of these options:
- `-ssh' selects the SSH protocol.
- `-telnet' selects the Telnet protocol.
- `-rlogin' selects the Rlogin protocol.
- `-raw' selects the raw protocol.
These options are not available in the file transfer tools PSCP and
PSFTP (which only work with the SSH protocol).
These options are equivalent to the protocol selection buttons
in the Session panel of the PuTTY configuration box (see section
4.1.1).
3.7.3.3 `-v': increase verbosity
Most of the PuTTY tools can be made to tell you more about what they
are doing by supplying the `-v' option. If you are having trouble
when making a connection, or you're simply curious, you can turn
this switch on and hope to find out more about what is happening.
3.7.3.4 `-l': specify a login name
You can specify the user name to log in as on the remote server
using the `-l' option. For example, `plink login.example.com -
l fred'.
These options are equivalent to the username selection box in
the Connection panel of the PuTTY configuration box (see section
4.14.1).
3.7.3.5 `-L', `-R' and `-D': set up port forwardings
As well as setting up port forwardings in the PuTTY configuration
(see section 4.22), you can also set up forwardings on the command
line. The command-line options work just like the ones in Unix `ssh'
programs.
To forward a local port (say 5110) to a remote destination (say
popserver.example.com port 110), you can write something like one of
these:
putty -L 5110:popserver.example.com:110 -load mysession
plink mysession -L 5110:popserver.example.com:110
To forward a remote port to a local destination, just use the `-R'
option instead of `-L':
putty -R 5023:mytelnetserver.myhouse.org:23 -load mysession
plink mysession -R 5023:mytelnetserver.myhouse.org:23
To specify an IP address for the listening end of the tunnel,
prepend it to the argument:
plink -L 127.0.0.5:23:localhost:23 myhost
To set up SOCKS-based dynamic port forwarding on a local port, use
the `-D' option. For this one you only have to pass the port number:
putty -D 4096 -load mysession
For general information on port forwarding, see section 3.5.
These options are not available in the file transfer tools PSCP and
PSFTP.
3.7.3.6 `-m': read a remote command or script from a file
The `-m' option performs a similar function to the `Remote command'
box in the SSH panel of the PuTTY configuration box (see section
4.18.1). However, the `-m' option expects to be given a local file
name, and it will read a command from that file. On most Unix
systems, you can even put multiple lines in this file and execute
more than one command in sequence, or a whole shell script; but this
will not work on all servers (and is known not to work with certain
`embedded' servers such as routers).
This option is not available in the file transfer tools PSCP and
PSFTP.
3.7.3.7 `-P': specify a port number
The `-P' option is used to specify the port number to connect to. If
you have a Telnet server running on port 9696 of a machine instead
of port 23, for example:
putty -telnet -P 9696 host.name
plink -telnet -P 9696 host.name
(Note that this option is more useful in Plink than in PuTTY,
because in PuTTY you can write `putty -telnet host.name 9696' in any
case.)
This option is equivalent to the port number control in the Session
panel of the PuTTY configuration box (see section 4.1.1).
3.7.3.8 `-pw': specify a password
A simple way to automate a remote login is to supply your password
on the command line. This is _not recommended_ for reasons of
security. If you possibly can, we recommend you set up public-key
authentication instead. See chapter 8 for details.
Note that the `-pw' option only works when you are using the SSH
protocol. Due to fundamental limitations of Telnet and Rlogin, these
protocols do not support automated password authentication.
3.7.3.9 `-A' and `-a': control agent forwarding
The `-A' option turns on SSH agent forwarding, and `-a' turns it
off. These options are only meaningful if you are using SSH.
See chapter 9 for general information on Pageant, and section 9.4
for information on agent forwarding. Note that there is a security
risk involved with enabling this option; see section 9.5 for
details.
These options are equivalent to the agent forwarding checkbox in the
Auth panel of the PuTTY configuration box (see section 4.20.3).
These options are not available in the file transfer tools PSCP and
PSFTP.
3.7.3.10 `-X' and `-x': control X11 forwarding
The `-X' option turns on X11 forwarding in SSH, and `-x' turns it
off. These options are only meaningful if you are using SSH.
For information on X11 forwarding, see section 3.4.
These options are equivalent to the X11 forwarding checkbox in the
Tunnels panel of the PuTTY configuration box (see section 4.21).
These options are not available in the file transfer tools PSCP and
PSFTP.
3.7.3.11 `-t' and `-T': control pseudo-terminal allocation
The `-t' option ensures PuTTY attempts to allocate a pseudo-terminal
at the server, and `-T' stops it from allocating one. These options
are only meaningful if you are using SSH.
These options are equivalent to the `Don't allocate a pseudo-
terminal' checkbox in the SSH panel of the PuTTY configuration box
(see section 4.18.2).
These options are not available in the file transfer tools PSCP and
PSFTP.
3.7.3.12 `-N': suppress starting a shell or command
The `-N' option prevents PuTTY from attempting to start a shell or
command on the remote server. You might want to use this option if
you are only using the SSH connection for port forwarding, and your
user account on the server does not have the ability to run a shell.
This feature is only available in SSH protocol version 2 (since the
version 1 protocol assumes you will always want to run a shell).
This option is equivalent to the `Don't start a shell or command at
all' checkbox in the SSH panel of the PuTTY configuration box (see
section 4.18.3).
This option is not available in the file transfer tools PSCP and
PSFTP.
3.7.3.13 `-C': enable compression
The `-C' option enables compression of the data sent across the
network. This option is only meaningful if you are using SSH.
This option is equivalent to the `Enable compression' checkbox in
the SSH panel of the PuTTY configuration box (see section 4.18.4).
3.7.3.14 `-1' and `-2': specify an SSH protocol version
The `-1' and `-2' options force PuTTY to use version 1 or version 2
of the SSH protocol. These options are only meaningful if you are
using SSH.
These options are equivalent to selecting your preferred SSH
protocol version as `1 only' or `2 only' in the SSH panel of the
PuTTY configuration box (see section 4.18.5).
3.7.3.15 `-4' and `-6': specify an Internet protocol version
The `-4' and `-6' options force PuTTY to use the older Internet
protocol IPv4 or the newer IPv6.
These options are equivalent to selecting your preferred Internet
protocol version as `IPv4' or `IPv6' in the Connection panel of the
PuTTY configuration box (see section 4.13.4).
3.7.3.16 `-i': specify an SSH private key
The `-i' option allows you to specify the name of a private key file
in `*.'`PPK' format which PuTTY will use to authenticate with the
server. This option is only meaningful if you are using SSH.
For general information on public-key authentication, see chapter 8.
This option is equivalent to the `Private key file for
authentication' box in the Auth panel of the PuTTY configuration box
(see section 4.20.5).
3.7.3.17 `-pgpfp': display PGP key fingerprints
This option causes the PuTTY tools not to run as normal, but instead
to display the fingerprints of the PuTTY PGP Master Keys, in
order to aid with verifying new versions. See appendix E for more
information.
Chapter 4: Configuring PuTTY
----------------------------
This chapter describes all the configuration options in PuTTY.
PuTTY is configured using the control panel that comes up before you
start a session. Some options can also be changed in the middle of a
session, by selecting `Change Settings' from the window menu.
4.1 The Session panel
The Session configuration panel contains the basic options you need
to specify in order to open a session at all, and also allows you to
save your settings to be reloaded later.
4.1.1 The host name section
The top box on the Session panel, labelled `Specify your connection
by host name', contains the details that need to be filled in before
PuTTY can open a session at all.
- The `Host Name' box is where you type the name, or the IP
address, of the server you want to connect to.
- The `Protocol' radio buttons let you choose what type of
connection you want to make: a raw connection, a Telnet
connection, an Rlogin connection or an SSH connection. (See
section 1.2 for a summary of the differences between SSH,
Telnet and rlogin, and section 3.6 for an explanation of `raw'
connections.)
- The `Port' box lets you specify which port number on the server
to connect to. If you select Telnet, Rlogin, or SSH, this box
will be filled in automatically to the usual value, and you will
only need to change it if you have an unusual server. If you
select Raw mode, you will almost certainly need to fill in the
`Port' box.
4.1.2 Loading and storing saved sessions
The next part of the Session configuration panel allows you to save
your preferred PuTTY options so they will appear automatically the
next time you start PuTTY. It also allows you to create _saved
sessions_, which contain a full set of configuration options plus a
host name and protocol. A saved session contains all the information
PuTTY needs to start exactly the session you want.
- To save your default settings: first set up the settings the way
you want them saved. Then come back to the Session panel. Select
the `Default Settings' entry in the saved sessions list, with a
single click. Then press the `Save' button.
Note that PuTTY does not allow you to save a host name into the
Default Settings entry. This ensures that when PuTTY is started
up, the host name box is always empty, so a user can always just
type in a host name and connect.
If there is a specific host you want to store the details of how
to connect to, you should create a saved session, which will be
separate from the Default Settings.
- To save a session: first go through the rest of the
configuration box setting up all the options you want. Then come
back to the Session panel. Enter a name for the saved session in
the `Saved Sessions' input box. (The server name is often a good
choice for a saved session name.) Then press the `Save' button.
Your saved session name should now appear in the list box.
You can also save settings in mid-session, from the `Change
Settings' dialog. Settings changed since the start of the
session will be saved with their current values; as well as
settings changed through the dialog, this includes changes in
window size, window title changes sent by the server, and so on.
- To reload a saved session: single-click to select the session
name in the list box, and then press the `Load' button. Your
saved settings should all appear in the configuration panel.
- To modify a saved session: first load it as described above.
Then make the changes you want. Come back to the Session panel,
and press the `Save' button. The new settings will be saved over
the top of the old ones.
To save the new settings under a different name, you can enter
the new name in the `Saved Sessions' box, or single-click to
select a session name in the list box to overwrite that session.
To save `Default Settings', you must single-click the name
before saving.
- To start a saved session immediately: double-click on the
session name in the list box.
- To delete a saved session: single-click to select the session
name in the list box, and then press the `Delete' button.
Each saved session is independent of the Default Settings
configuration. If you change your preferences and update Default
Settings, you must also update every saved session separately.
Saved sessions are stored in the Registry, at the location
HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\Sessions
If you need to store them in a file, you could try the method
described in section 4.24.
4.1.3 `Close Window on Exit'
Finally in the Session panel, there is an option labelled `Close
Window on Exit'. This controls whether the PuTTY terminal window
disappears as soon as the session inside it terminates. If you are
likely to want to copy and paste text out of the session after it
has terminated, or restart the session, you should arrange for this
option to be off.
`Close Window On Exit' has three settings. `Always' means always
close the window on exit; `Never' means never close on exit (always
leave the window open, but inactive). The third setting, and the
default one, is `Only on clean exit'. In this mode, a session which
terminates normally will cause its window to close, but one which is
aborted unexpectedly by network trouble or a confusing message from
the server will leave the window up.
4.2 The Logging panel
The Logging configuration panel allows you to save log files of your
PuTTY sessions, for debugging, analysis or future reference.
The main option is a radio-button set that specifies whether PuTTY
will log anything at all. The options are
- `Logging turned off completely'. This is the default option; in
this mode PuTTY will not create a log file at all.
- `Log printable output only'. In this mode, a log file will be
created and written to, but only printable text will be saved
into it. The various terminal control codes that are typically
sent down an interactive session alongside the printable text
will be omitted. This might be a useful mode if you want to read
a log file in a text editor and hope to be able to make sense of
it.
- `Log all session output'. In this mode, _everything_ sent by the
server into your terminal session is logged. If you view the log
file in a text editor, therefore, you may well find it full of
strange control characters. This is a particularly useful mode
if you are experiencing problems with PuTTY's terminal handling:
you can record everything that went to the terminal, so that
someone else can replay the session later in slow motion and
watch to see what went wrong.
- `Log SSH packet data'. In this mode (which is only used by SSH
connections), the SSH message packets sent over the encrypted
connection are written to the log file. You might need this to
debug a network-level problem, or more likely to send to the
PuTTY authors as part of a bug report. _BE WARNED_ that if you
log in using a password, the password can appear in the log
file; see section 4.2.4 for options that may help to remove
sensitive material from the log file before you send it to
anyone else.
4.2.1 `Log file name'
In this edit box you enter the name of the file you want to log the
session to. The `Browse' button will let you look around your file
system to find the right place to put the file; or if you already
know exactly where you want it to go, you can just type a pathname
into the edit box.
There are a few special features in this box. If you use the `&'
character in the file name box, PuTTY will insert details of the
current session in the name of the file it actually opens. The
precise replacements it will do are:
- `&Y' will be replaced by the current year, as four digits.
- `&M' will be replaced by the current month, as two digits.
- `&D' will be replaced by the current day of the month, as two
digits.
- `&T' will be replaced by the current time, as six digits
(HHMMSS) with no punctuation.
- `&H' will be replaced by the host name you are connecting to.
For example, if you enter the host name `c:\puttylogs\log-&h-&y&m&d-
&t.dat', you will end up with files looking like
log-server1.example.com-20010528-110859.dat
log-unixbox.somewhere.org-20010611-221001.dat
4.2.2 `What to do if the log file already exists'
This control allows you to specify what PuTTY should do if it tries
to start writing to a log file and it finds the file already exists.
You might want to automatically destroy the existing log file and
start a new one with the same name. Alternatively, you might want to
open the existing log file and add data to the _end_ of it. Finally
(the default option), you might not want to have any automatic
behaviour, but to ask the user every time the problem comes up.
4.2.3 `Flush log file frequently'
This option allows you to control how frequently logged data is
flushed to disc. By default, PuTTY will flush data as soon as it
is displayed, so that if you view the log file while a session is
still open, it will be up to date; and if the client system crashes,
there's a greater chance that the data will be preserved.
However, this can incur a performance penalty. If PuTTY is running
slowly with logging enabled, you could try unchecking this option.
Be warned that the log file may not always be up to date as a result
(although it will of course be flushed when it is closed, for
instance at the end of a session).
4.2.4 Options specific to SSH packet logging
These options only apply if SSH packet data is being logged.
The following options allow particularly sensitive portions of
unencrypted packets to be automatically left out of the log file.
They are only intended to deter casual nosiness; an attacker could
glean a lot of useful information from even these obfuscated logs
(e.g., length of password).
4.2.4.1 `Omit known password fields'
When checked, password fields are removed from the log of
transmitted packets. (This includes any user responses to challenge-
response authentication methods such as `keyboard-interactive'.)
This does not include X11 authentication data if using X11
forwarding.
Note that this will only omit data that PuTTY _knows_ to be a
password. However, if you start another login session within your
PuTTY session, for instance, any password used will appear in the
clear in the packet log. The next option may be of use to protect
against this.
This option is enabled by default.
4.2.4.2 `Omit session data'
When checked, all `session data' is omitted; this is defined as
data in terminal sessions and in forwarded channels (TCP, X11, and
authentication agent). This will usually substantially reduce the
size of the resulting log file.
This option is disabled by default.
4.3 The Terminal panel
The Terminal configuration panel allows you to control the behaviour
of PuTTY's terminal emulation.
4.3.1 `Auto wrap mode initially on'
Auto wrap mode controls what happens when text printed in a PuTTY
window reaches the right-hand edge of the window.
With auto wrap mode on, if a long line of text reaches the right-
hand edge, it will wrap over on to the next line so you can still
see all the text. With auto wrap mode off, the cursor will stay at
the right-hand edge of the screen, and all the characters in the
line will be printed on top of each other.
If you are running a full-screen application and you occasionally
find the screen scrolling up when it looks as if it shouldn't, you
could try turning this option off.
Auto wrap mode can be turned on and off by control sequences sent by
the server. This configuration option controls the _default_ state,
which will be restored when you reset the terminal (see section 3.
1.3.6). However, if you modify this option in mid-session using
`Change Settings', it will take effect immediately.
4.3.2 `DEC Origin Mode initially on'
DEC Origin Mode is a minor option which controls how PuTTY
interprets cursor-position control sequences sent by the server.
The server can send a control sequence that restricts the scrolling
region of the display. For example, in an editor, the server might
reserve a line at the top of the screen and a line at the bottom,
and might send a control sequence that causes scrolling operations
to affect only the remaining lines.
With DEC Origin Mode on, cursor coordinates are counted from the top
of the scrolling region. With it turned off, cursor coordinates are
counted from the top of the whole screen regardless of the scrolling
region.
It is unlikely you would need to change this option, but if you find
a full-screen application is displaying pieces of text in what looks
like the wrong part of the screen, you could try turning DEC Origin
Mode on to see whether that helps.
DEC Origin Mode can be turned on and off by control sequences sent
by the server. This configuration option controls the _default_
state, which will be restored when you reset the terminal (see
section 3.1.3.6). However, if you modify this option in mid-session
using `Change Settings', it will take effect immediately.
4.3.3 `Implicit CR in every LF'
Most servers send two control characters, CR and LF, to start a new
line of the screen. The CR character makes the cursor return to the
left-hand side of the screen. The LF character makes the cursor move
one line down (and might make the screen scroll).
Some servers only send LF, and expect the terminal to move the
cursor over to the left automatically. If you come across a server
that does this, you will see a stepped effect on the screen, like
this:
First line of text
Second line
Third line
If this happens to you, try enabling the `Implicit CR in every LF'
option, and things might go back to normal:
First line of text
Second line
Third line
4.3.4 `Use background colour to erase screen'
Not all terminals agree on what colour to turn the screen when the
server sends a `clear screen' sequence. Some terminals believe the
screen should always be cleared to the _default_ background colour.
Others believe the screen should be cleared to whatever the server
has selected as a background colour.
There exist applications that expect both kinds of behaviour.
Therefore, PuTTY can be configured to do either.
With this option disabled, screen clearing is always done in the
default background colour. With this option enabled, it is done in
the _current_ background colour.
Background-colour erase can be turned on and off by control
sequences sent by the server. This configuration option controls the
_default_ state, which will be restored when you reset the terminal
(see section 3.1.3.6). However, if you modify this option in mid-
session using `Change Settings', it will take effect immediately.
4.3.5 `Enable blinking text'
The server can ask PuTTY to display text that blinks on and off.
This is very distracting, so PuTTY allows you to turn blinking text
off completely.
When blinking text is disabled and the server attempts to make
some text blink, PuTTY will instead display the text with a bolded
background colour.
Blinking text can be turned on and off by control sequences sent by
the server. This configuration option controls the _default_ state,
which will be restored when you reset the terminal (see section 3.
1.3.6). However, if you modify this option in mid-session using
`Change Settings', it will take effect immediately.
4.3.6 `Answerback to ^E'
This option controls what PuTTY will send back to the server if the
server sends it the ^E enquiry character. Normally it just sends the
string `PuTTY'.
If you accidentally write the contents of a binary file to your
terminal, you will probably find that it contains more than one ^E
character, and as a result your next command line will probably
read `PuTTYPuTTYPuTTY...' as if you had typed the answerback string
multiple times at the keyboard. If you set the answerback string
to be empty, this problem should go away, but doing so might cause
other problems.
Note that this is _not_ the feature of PuTTY which the server will
typically use to determine your terminal type. That feature is the
`Terminal-type string' in the Connection panel; see section 4.14.2
for details.
You can include control characters in the answerback string using
`^C' notation. (Use `^~' to get a literal `^'.)
4.3.7 `Local echo'
With local echo disabled, characters you type into the PuTTY window
are not echoed in the window _by PuTTY_. They are simply sent to the
server. (The _server_ might choose to echo them back to you; this
can't be controlled from the PuTTY control panel.)
Some types of session need local echo, and many do not. In its
default mode, PuTTY will automatically attempt to deduce whether
or not local echo is appropriate for the session you are working
in. If you find it has made the wrong decision, you can use this
configuration option to override its choice: you can force local
echo to be turned on, or force it to be turned off, instead of
relying on the automatic detection.
4.3.8 `Local line editing'
Normally, every character you type into the PuTTY window is sent
immediately to the server the moment you type it.
If you enable local line editing, this changes. PuTTY will let you
edit a whole line at a time locally, and the line will only be sent
to the server when you press Return. If you make a mistake, you can
use the Backspace key to correct it before you press Return, and the
server will never see the mistake.
Since it is hard to edit a line locally without being able to see
it, local line editing is mostly used in conjunction with local echo
(section 4.3.7). This makes it ideal for use in raw mode or when
connecting to MUDs or talkers. (Although some more advanced MUDs do
occasionally turn local line editing on and turn local echo off, in
order to accept a password from the user.)
Some types of session need local line editing, and many do not. In
its default mode, PuTTY will automatically attempt to deduce whether
or not local line editing is appropriate for the session you are
working in. If you find it has made the wrong decision, you can use
this configuration option to override its choice: you can force
local line editing to be turned on, or force it to be turned off,
instead of relying on the automatic detection.
4.3.9 Remote-controlled printing
A lot of VT100-compatible terminals support printing under control
of the remote server. PuTTY supports this feature as well, but it is
turned off by default.
To enable remote-controlled printing, choose a printer from
the `Printer to send ANSI printer output to' drop-down list
box. This should allow you to select from all the printers you
have installed drivers for on your computer. Alternatively, you
can type the network name of a networked printer (for example,
`\\printserver\printer1') even if you haven't already installed a
driver for it on your own machine.
When the remote server attempts to print some data, PuTTY will send
that data to the printer _raw_ - without translating it, attempting
to format it, or doing anything else to it. It is up to you to
ensure your remote server knows what type of printer it is talking
to.
Since PuTTY sends data to the printer raw, it cannot offer options
such as portrait versus landscape, print quality, or paper tray
selection. All these things would be done by your PC printer driver
(which PuTTY bypasses); if you need them done, you will have to find
a way to configure your remote server to do them.
To disable remote printing again, choose `None (printing disabled)'
from the printer selection list. This is the default state.
4.4 The Keyboard panel
The Keyboard configuration panel allows you to control the behaviour
of the keyboard in PuTTY.
4.4.1 Changing the action of the Backspace key
Some terminals believe that the Backspace key should send the same
thing to the server as Control-H (ASCII code 8). Other terminals
believe that the Backspace key should send ASCII code 127 (usually
known as Control-?) so that it can be distinguished from Control-H.
This option allows you to choose which code PuTTY generates when you
press Backspace.
If you are connecting to a Unix system, you will probably find that
the Unix `stty' command lets you configure which the server expects
to see, so you might not need to change which one PuTTY generates.
On other systems, the server's expectation might be fixed and you
might have no choice but to configure PuTTY.
If you do have the choice, we recommend configuring PuTTY to
generate Control-? and configuring the server to expect it, because
that allows applications such as `emacs' to use Control-H for help.
(Typing Shift-Backspace will cause PuTTY to send whichever code
isn't configured here as the default.)
4.4.2 Changing the action of the
The Unix terminal emulator `rxvt' disagrees with the rest of the
world about what character sequences should be sent to the server by
the Home and End keys.
`xterm', and other terminals, send `ESC [1~' for the Home key, and
`ESC [4~' for the End key. `rxvt' sends `ESC [H' for the Home key
and `ESC [Ow' for the End key.
If you find an application on which the Home and End keys aren't
working, you could try switching this option to see if it helps.
4.4.3 Changing the action of the function keys and keypad
This option affects the function keys (F1 to F12) and the top row of
the numeric keypad.
- In the default mode, labelled `ESC [n~', the function keys
generate sequences like `ESC [11~', `ESC [12~' and so on. This
matches the general behaviour of Digital's terminals.
- In Linux mode, F6 to F12 behave just like the default mode, but
F1 to F5 generate `ESC [[A' through to `ESC [[E'. This mimics
the Linux virtual console.
- In Xterm R6 mode, F5 to F12 behave like the default mode, but
F1 to F4 generate `ESC OP' through to `ESC OS', which are the
sequences produced by the top row of the _keypad_ on Digital's
terminals.
- In VT400 mode, all the function keys behave like the default
mode, but the actual top row of the numeric keypad generates
`ESC OP' through to `ESC OS'.
- In VT100+ mode, the function keys generate `ESC OP' through to
`ESC O['
- In SCO mode, the function keys F1 to F12 generate `ESC [M'
through to `ESC [X'. Together with shift, they generate `ESC [Y'
through to `ESC [j'. With control they generate `ESC [k' through
to `ESC [v', and with shift and control together they generate
`ESC [w' through to `ESC [{'.
If you don't know what any of this means, you probably don't need to
fiddle with it.
4.4.4 Controlling Application Cursor Keys mode
Application Cursor Keys mode is a way for the server to change the
control sequences sent by the arrow keys. In normal mode, the arrow
keys send `ESC [A' through to `ESC [D'. In application mode, they
send `ESC OA' through to `ESC OD'.
Application Cursor Keys mode can be turned on and off by the server,
depending on the application. PuTTY allows you to configure the
initial state.
You can also disable application cursor keys mode completely, using
the `Features' configuration panel; see section 4.6.1.
4.4.5 Controlling Application Keypad mode
Application Keypad mode is a way for the server to change the
behaviour of the numeric keypad.
In normal mode, the keypad behaves like a normal Windows keypad:
with NumLock on, the number keys generate numbers, and with NumLock
off they act like the arrow keys and Home, End etc.
In application mode, all the keypad keys send special control
sequences, _including_ Num Lock. Num Lock stops behaving like Num
Lock and becomes another function key.
Depending on which version of Windows you run, you may find the Num
Lock light still flashes on and off every time you press Num Lock,
even when application mode is active and Num Lock is acting like a
function key. This is unavoidable.
Application keypad mode can be turned on and off by the server,
depending on the application. PuTTY allows you to configure the
initial state.
You can also disable application keypad mode completely, using the
`Features' configuration panel; see section 4.6.1.
4.4.6 Using NetHack keypad mode
PuTTY has a special mode for playing NetHack. You can enable it
by selecting `NetHack' in the `Initial state of numeric keypad'
control.
In this mode, the numeric keypad keys 1-9 generate the NetHack
movement commands (hjklyubn). The 5 key generates the `.' command
(do nothing).
Better still, pressing Shift with the keypad keys generates the
capital forms of the commands (HJKLYUBN), which tells NetHack to
keep moving you in the same direction until you encounter something
interesting.
For some reason, this feature only works properly when Num Lock is
on. We don't know why.
4.4.7 Enabling a DEC-like Compose key
DEC terminals have a Compose key, which provides an easy-to-remember
way of typing accented characters. You press Compose and then type
two more characters. The two characters are `combined' to produce
an accented character. The choices of character are designed to be
easy to remember; for example, composing `e' and ``' produces the
`e-grave' character.
If your keyboard has a Windows Application key, it acts as a Compose
key in PuTTY. Alternatively, if you enable the `AltGr acts as
Compose key' option, the AltGr key will become a Compose key.
4.4.8 `Control-Alt is different from AltGr'
Some old keyboards do not have an AltGr key, which can make it
difficult to type some characters. PuTTY can be configured to treat
the key combination Ctrl + Left Alt the same way as the AltGr key.
By default, this checkbox is checked, and the key combination Ctrl +
Left Alt does something completely different. PuTTY's usual handling
of the left Alt key is to prefix the Escape (Control-[) character to
whatever character sequence the rest of the keypress would generate.
For example, Alt-A generates Escape followed by `a'. So Alt-Ctrl-A
would generate Escape, followed by Control-A.
If you uncheck this box, Ctrl-Alt will become a synonym for AltGr,
so you can use it to type extra graphic characters if your keyboard
has any.
(However, Ctrl-Alt will never act as a Compose key, regardless of
the setting of `AltGr acts as Compose key' described in section
4.4.7.)
4.5 The Bell panel
The Bell panel controls the terminal bell feature: the server's
ability to cause PuTTY to beep at you.
In the default configuration, when the server sends the character
with ASCII code 7 (Control-G), PuTTY will play the Windows Default
Beep sound. This is not always what you want the terminal bell
feature to do; the Bell panel allows you to configure alternative
actions.
4.5.1 `Set the style of bell'
This control allows you to select various different actions to occur
on a terminal bell:
- Selecting `None' disables the bell completely. In this mode, the
server can send as many Control-G characters as it likes and
nothing at all will happen.
- `Make default system alert sound' is the default setting.
It causes the Windows `Default Beep' sound to be played. To
change what this sound is, or to test it if nothing seems to
be happening, use the Sound configurer in the Windows Control
Panel.
- `Visual bell' is a silent alternative to a beeping computer. In
this mode, when the server sends a Control-G, the whole PuTTY
window will flash white for a fraction of a second.
- `Beep using the PC speaker' is self-explanatory.
- `Play a custom sound file' allows you to specify a particular
sound file to be used by PuTTY alone, or even by a particular
individual PuTTY session. This allows you to distinguish your
PuTTY beeps from any other beeps on the system. If you select
this option, you will also need to enter the name of your sound
file in the edit control `Custom sound file to play as a bell'.
4.5.2 `Taskbar/caption indication on bell'
This feature controls what happens to the PuTTY window's entry in
the Windows Taskbar if a bell occurs while the window does not have
the input focus.
In the default state (`Disabled') nothing unusual happens.
If you select `Steady', then when a bell occurs and the window is
not in focus, the window's Taskbar entry and its title bar will
change colour to let you know that PuTTY session is asking for your
attention. The change of colour will persist until you select the
window, so you can leave several PuTTY windows minimised in your
terminal, go away from your keyboard, and be sure not to have missed
any important beeps when you get back.
`Flashing' is even more eye-catching: the Taskbar entry will
continuously flash on and off until you select the window.
4.5.3 `Control the bell overload behaviour'
A common user error in a terminal session is to accidentally run the
Unix command `cat' (or equivalent) on an inappropriate file type,
such as an executable, image file, or ZIP file. This produces a huge
stream of non-text characters sent to the terminal, which typically
includes a lot of bell characters. As a result of this the terminal
often doesn't stop beeping for ten minutes, and everybody else in
the office gets annoyed.
To try to avoid this behaviour, or any other cause of excessive
beeping, PuTTY includes a bell overload management feature. In the
default configuration, receiving more than five bell characters in a
two-second period will cause the overload feature to activate. Once
the overload feature is active, further bells will have no effect
at all, so the rest of your binary file will be sent to the screen
in silence. After a period of five seconds during which no further
bells are received, the overload feature will turn itself off again
and bells will be re-enabled.
If you want this feature completely disabled, you can turn it off
using the checkbox `Bell is temporarily disabled when over-used'.
Alternatively, if you like the bell overload feature but don't
agree with the settings, you can configure the details: how many
bells constitute an overload, how short a time period they have to
arrive in to do so, and how much silent time is required before the
overload feature will deactivate itself.
Bell overload mode is always deactivated by any keypress in the
terminal. This means it can respond to large unexpected streams of
data, but does not interfere with ordinary command-line activities
that generate beeps (such as filename completion).
4.6 The Features panel
PuTTY's terminal emulation is very highly featured, and can do a
lot of things under remote server control. Some of these features
can cause problems due to buggy or strangely configured server
applications.
The Features configuration panel allows you to disable some of
PuTTY's more advanced terminal features, in case they cause trouble.
4.6.1 Disabling application keypad and cursor keys
Application keypad mode (see section 4.4.5) and application cursor
keys mode (see section 4.4.4) alter the behaviour of the keypad and
cursor keys. Some applications enable these modes but then do not
deal correctly with the modified keys. You can force these modes to
be permanently disabled no matter what the server tries to do.
4.6.2 Disabling xterm-style mouse reporting
PuTTY allows the server to send control codes that let it take
over the mouse and use it for purposes other than copy and paste.
Applications which use this feature include the text-mode web
browser `links', the Usenet newsreader `trn' version 4, and the file
manager `mc' (Midnight Commander).
If you find this feature inconvenient, you can disable it using the
`Disable xterm-style mouse reporting' control. With this box ticked,
the mouse will _always_ do copy and paste in the normal way.
Note that even if the application takes over the mouse, you can
still manage PuTTY's copy and paste by holding down the Shift key
while you select and paste, unless you have deliberately turned this
feature off (see section 4.11.3).
4.6.3 Disabling remote terminal resizing
PuTTY has the ability to change the terminal's size and position in
response to commands from the server. If you find PuTTY is doing
this unexpectedly or inconveniently, you can tell PuTTY not to
respond to those server commands.
4.6.4 Disabling switching to the alternate screen
Many terminals, including PuTTY, support an `alternate screen'. This
is the same size as the ordinary terminal screen, but separate.
Typically a screen-based program such as a text editor might switch
the terminal to the alternate screen before starting up. Then at the
end of the run, it switches back to the primary screen, and you see
the screen contents just as they were before starting the editor.
Some people prefer this not to happen. If you want your editor to
run in the same screen as the rest of your terminal activity, you
can disable the alternate screen feature completely.
4.6.5 Disabling remote window title changing
PuTTY has the ability to change the window title in response
to commands from the server. If you find PuTTY is doing this
unexpectedly or inconveniently, you can tell PuTTY not to respond to
those server commands.
4.6.6 Disabling remote window title querying
PuTTY can optionally provide the xterm service of allowing server
applications to find out the local window title. This feature is
disabled by default, but you can turn it on if you really want it.
NOTE that this feature is a _potential __security hazard_. If a
malicious application can write data to your terminal (for example,
if you merely `cat' a file owned by someone else on the server
machine), it can change your window title (unless you have disabled
this as mentioned in section 4.6.5) and then use this service to
have the new window title sent back to the server as if typed at the
keyboard. This allows an attacker to fake keypresses and potentially
cause your server-side applications to do things you didn't want.
Therefore this feature is disabled by default, and we recommend you
do not turn it on unless you _really_ know what you are doing.
4.6.7 Disabling destructive backspace
Normally, when PuTTY receives character 127 (^?) from the server,
it will perform a `destructive backspace': move the cursor one
space left and delete the character under it. This can apparently
cause problems in some applications, so PuTTY provides the ability
to configure character 127 to perform a normal backspace (without
deleting a character) instead.
4.6.8 Disabling remote character set configuration
PuTTY has the ability to change its character set configuration
in response to commands from the server. Some programs send these
commands unexpectedly or inconveniently. In particular, (an IRC
client) seems to have a habit of reconfiguring the character set to
something other than the user intended.
If you find that accented characters are not showing up the way you
expect them to, particularly if you're running BitchX, you could try
disabling the remote character set configuration commands.
4.6.9 Disabling Arabic text shaping
PuTTY supports shaping of Arabic text, which means that if your
server sends text written in the basic Unicode Arabic alphabet then
it will convert it to the correct display forms before printing it
on the screen.
If you are using full-screen software which was not expecting
this to happen (especially if you are not an Arabic speaker and
you unexpectedly find yourself dealing with Arabic text files in
applications which are not Arabic-aware), you might find that the
display becomes corrupted. By ticking this box, you can disable
Arabic text shaping so that PuTTY displays precisely the characters
it is told to display.
You may also find you need to disable bidirectional text display;
see section 4.6.10.
4.6.10 Disabling bidirectional text display
PuTTY supports bidirectional text display, which means that if your
server sends text written in a language which is usually displayed
from right to left (such as Arabic or Hebrew) then PuTTY will
automatically flip it round so that it is displayed in the right
direction on the screen.
If you are using full-screen software which was not expecting
this to happen (especially if you are not an Arabic speaker and
you unexpectedly find yourself dealing with Arabic text files in
applications which are not Arabic-aware), you might find that the
display becomes corrupted. By ticking this box, you can disable
bidirectional text display, so that PuTTY displays text from left to
right in all situations.
You may also find you need to disable Arabic text shaping; see
section 4.6.9.
4.7 The Window panel
The Window configuration panel allows you to control aspects of the
PuTTY window.
4.7.1 Setting the size of the PuTTY window
The `Rows' and `Columns' boxes let you set the PuTTY window to a
precise size. Of course you can also drag the window to a new size
while a session is running.
4.7.2 What to do when the window is resized
These options allow you to control what happens when the user tries
to resize the PuTTY window using its window furniture.
There are four options here:
- `Change the number of rows and columns': the font size will not
change. (This is the default.)
- `Change the size of the font': the number of rows and columns in
the terminal will stay the same, and the font size will change.
- `Change font size when maximised': when the window is resized,
the number of rows and columns will change, _except_ when the
window is maximised (or restored), when the font size will
change.
- `Forbid resizing completely': the terminal will refuse to be
resized at all.
4.7.3 Controlling scrollback
These options let you configure the way PuTTY keeps text after it
scrolls off the top of the screen (see section 3.1.2).
The `Lines of scrollback' box lets you configure how many lines of
text PuTTY keeps. The `Display scrollbar' options allow you to hide
the scrollbar (although you can still view the scrollback using
the keyboard as described in section 3.1.2). You can separately
configure whether the scrollbar is shown in full-screen mode and in
normal modes.
If you are viewing part of the scrollback when the server sends
more text to PuTTY, the screen will revert to showing the current
terminal contents. You can disable this behaviour by turning off
`Reset scrollback on display activity'. You can also make the screen
revert when you press a key, by turning on `Reset scrollback on
keypress'.
4.7.4 `Push erased text into scrollback'
When this option is enabled, the contents of the terminal screen
will be pushed into the scrollback when a server-side application
clears the screen, so that your scrollback will contain a better
record of what was on your screen in the past.
If the application switches to the alternate screen (see section
4.6.4 for more about this), then the contents of the primary screen
will be visible in the scrollback until the application switches
back again.
This option is enabled by default.
4.8 The Appearance panel
The Appearance configuration panel allows you to control aspects of
the appearance of PuTTY's window.
4.8.1 Controlling the appearance of the cursor
The `Cursor appearance' option lets you configure the cursor to be a
block, an underline, or a vertical line. A block cursor becomes an
empty box when the window loses focus; an underline or a vertical
line becomes dotted.
The `Cursor blinks' option makes the cursor blink on and off. This
works in any of the cursor modes.
4.8.2 Controlling the font used in the terminal window
This option allows you to choose what font, in what size, the PuTTY
terminal window uses to display the text in the session. You will
be offered a choice from all the fixed-width fonts installed on the
system. (VT100-style terminal handling can only deal with fixed-
width fonts.)
4.8.3 `Hide mouse pointer when typing in window'
If you enable this option, the mouse pointer will disappear if the
PuTTY window is selected and you press a key. This way, it will
not obscure any of the text in the window while you work in your
session. As soon as you move the mouse, the pointer will reappear.
This option is disabled by default, so the mouse pointer remains
visible at all times.
4.8.4 Controlling the window border
PuTTY allows you to configure the appearance of the window border to
some extent.
The checkbox marked `Sunken-edge border' changes the appearance of
the window border to something more like a DOS box: the inside edge
of the border is highlighted as if it sank down to meet the surface
inside the window. This makes the border a little bit thicker as
well. It's hard to describe well. Try it and see if you like it.
You can also configure a completely blank gap between the text in
the window and the border, using the `Gap between text and window
edge' control. By default this is set at one pixel. You can reduce
it to zero, or increase it further.
4.9 The Behaviour panel
The Behaviour configuration panel allows you to control aspects of
the behaviour of PuTTY's window.
4.9.1 Controlling the window title
The `Window title' edit box allows you to set the title of the PuTTY
window. By default the window title will contain the host name
followed by `PuTTY', for example `server1.example.com - PuTTY'. If
you want a different window title, this is where to set it.
PuTTY allows the server to send `xterm' control sequences which
modify the title of the window in mid-session (unless this is
disabled - see section 4.6.5); the title string set here is
therefore only the _initial_ window title.
As well as the _window_ title, there is also an `xterm' sequence
to modify the title of the window's _icon_. This makes sense in a
windowing system where the window becomes an icon when minimised,
such as Windows 3.1 or most X Window System setups; but in the
Windows 95-like user interface it isn't as applicable.
By default, PuTTY only uses the server-supplied _window_ title,
and ignores the icon title entirely. If for some reason you want
to see both titles, check the box marked `Separate window and icon
titles'. If you do this, PuTTY's window title and Taskbar caption
will change into the server-supplied icon title if you minimise the
PuTTY window, and change back to the server-supplied window title if
you restore it. (If the server has not bothered to supply a window
or icon title, none of this will happen.)
4.9.2 `Warn before closing window'
If you press the Close button in a PuTTY window that contains a
running session, PuTTY will put up a warning window asking if you
really meant to close the window. A window whose session has already
terminated can always be closed without a warning.
If you want to be able to close a window quickly, you can disable
the `Warn before closing window' option.
4.9.3 `Window closes on ALT-F4'
By default, pressing ALT-F4 causes the window to close (or a warning
box to appear; see section 4.9.2). If you disable the `Window closes
on ALT-F4' option, then pressing ALT-F4 will simply send a key
sequence to the server.
4.9.4 `System menu appears on ALT-Space'
If this option is enabled, then pressing ALT-Space will bring up the
PuTTY window's menu, like clicking on the top left corner. If it is
disabled, then pressing ALT-Space will just send `ESC SPACE' to the
server.
Some accessibility programs for Windows may need this option
enabling to be able to control PuTTY's window successfully. For
instance, Dragon NaturallySpeaking requires it both to open the
system menu via voice, and to close, minimise, maximise and restore
the window.
4.9.5 `System menu appears on Alt alone'
If this option is enabled, then pressing and releasing ALT will
bring up the PuTTY window's menu, like clicking on the top left
corner. If it is disabled, then pressing and releasing ALT will have
no effect.
4.9.6 `Ensure window is always on top'
If this option is enabled, the PuTTY window will stay on top of all
other windows.
4.9.7 `Full screen on Alt-Enter'
If this option is enabled, then pressing Alt-Enter will cause the
PuTTY window to become full-screen. Pressing Alt-Enter again will
restore the previous window size.
The full-screen feature is also available from the System menu, even
when it is configured not to be available on the Alt-Enter key. See
section 3.1.3.7.
4.10 The Translation panel
The Translation configuration panel allows you to control the
translation between the character set understood by the server and
the character set understood by PuTTY.
4.10.1 Controlling character set translation
During an interactive session, PuTTY receives a stream of 8-bit
bytes from the server, and in order to display them on the screen it
needs to know what character set to interpret them in.
There are a lot of character sets to choose from. The `Received data
assumed to be in which character set' option lets you select one. By
default PuTTY will attempt to choose a character set that is right
for your locale as reported by Windows; if it gets it wrong, you can
select a different one using this control.
A few notable character sets are:
- The ISO-8859 series are all standard character sets that include
various accented characters appropriate for different sets of
languages.
- The Win125x series are defined by Microsoft, for similar
purposes. In particular Win1252 is almost equivalent to ISO-
8859-1, but contains a few extra characters such as matched
quotes and the Euro symbol.
- If you want the old IBM PC character set with block graphics and
line-drawing characters, you can select `CP437'.
- PuTTY also supports Unicode mode, in which the data coming from
the server is interpreted as being in the UTF-8 encoding of
Unicode. If you select `UTF-8' as a character set you can use
this mode. Not all server-side applications will support it.
If you need support for a numeric code page which is not listed in
the drop-down list, such as code page 866, then you can try entering
its name manually (`CP866' for example) in the list box. If the
underlying version of Windows has the appropriate translation table
installed, PuTTY will use it.
4.10.2 `Treat CJK ambiguous characters as wide'
There are some Unicode characters whose width is not well-defined.
In most contexts, such characters should be treated as single-
width for the purposes of wrapping and so on; however, in some CJK
contexts, they are better treated as double-width for historical
reasons, and some server-side applications may expect them to be
displayed as such. Setting this option will cause PuTTY to take the
double-width interpretation.
If you use legacy CJK applications, and you find your lines are
wrapping in the wrong places, or you are having other display
problems, you might want to play with this setting.
This option only has any effect in UTF-8 mode (see section 4.10.1).
4.10.3 `Caps Lock acts as Cyrillic switch'
This feature allows you to switch between a US/UK keyboard layout
and a Cyrillic keyboard layout by using the Caps Lock key, if you
need to type (for example) Russian and English side by side in the
same document.
Currently this feature is not expected to work properly if your
native keyboard layout is not US or UK.
4.10.4 Controlling display of line-drawing characters
VT100-series terminals allow the server to send control sequences
that shift temporarily into a separate character set for drawing
simple lines and boxes. However, there are a variety of ways in
which PuTTY can attempt to find appropriate characters, and the
right one to use depends on the locally configured font. In general
you should probably try lots of options until you find one that your
particular font supports.
- `Use Unicode line drawing code points' tries to use the box
characters that are present in Unicode. For good Unicode-
supporting fonts this is probably the most reliable and
functional option.
- `Poor man's line drawing' assumes that the font _cannot_
generate the line and box characters at all, so it will use the
`+', `-' and `|' characters to draw approximations to boxes. You
should use this option if none of the other options works.
- `Font has XWindows encoding' is for use with fonts that have a
special encoding, where the lowest 32 character positions (below
the ASCII printable range) contain the line-drawing characters.
This is unlikely to be the case with any standard Windows font;
it will probably only apply to custom-built fonts or fonts that
have been automatically converted from the X Window System.
- `Use font in both ANSI and OEM modes' tries to use the same font
in two different character sets, to obtain a wider range of
characters. This doesn't always work; some fonts claim to be a
different size depending on which character set you try to use.
- `Use font in OEM mode only' is more reliable than that, but can
miss out other characters from the main character set.
4.10.5 Controlling copy and paste of line drawing characters
By default, when you copy and paste a piece of the PuTTY screen that
contains VT100 line and box drawing characters, PuTTY will paste
them in the form they appear on the screen: either Unicode line
drawing code points, or the `poor man's' line-drawing characters
`+', `-' and `|'. The checkbox `Copy and paste VT100 line drawing
chars as lqqqk' disables this feature, so line-drawing characters
will be pasted as the ASCII characters that were printed to produce
them. This will typically mean they come out mostly as `q' and `x',
with a scattering of `jklmntuvw' at the corners. This might be
useful if you were trying to recreate the same box layout in another
program, for example.
Note that this option only applies to line-drawing characters which
_were_ printed by using the VT100 mechanism. Line-drawing characters
that were received as Unicode code points will paste as Unicode
always.
4.11 The Selection panel
The Selection panel allows you to control the way copy and paste
work in the PuTTY window.
4.11.1 Pasting in Rich Text Format
If you enable `Paste to clipboard in RTF as well as plain text',
PuTTY will write formatting information to the clipboard as well as
the actual text you copy. Currently the only effect of this will be
that if you paste into (say) a word processor, the text will appear
in the word processor in the same font PuTTY was using to display
it. In future it is likely that other formatting information (bold,
underline, colours) will be copied as well.
This option can easily be inconvenient, so by default it is
disabled.
4.11.2 Changing the actions of the mouse buttons
PuTTY's copy and paste mechanism is by default modelled on the Unix
`xterm' application. The X Window System uses a three-button mouse,
and the convention is that the left button selects, the right button
extends an existing selection, and the middle button pastes.
Windows often only has two mouse buttons, so in PuTTY's default
configuration (`Compromise'), the _right_ button pastes, and the
_middle_ button (if you have one) extends a selection.
If you have a three-button mouse and you are already used to the
`xterm' arrangement, you can select it using the `Action of mouse
buttons' control.
Alternatively, with the `Windows' option selected, the middle button
extends, and the right button brings up a context menu (on which one
of the options is `Paste'). (This context menu is always available
by holding down Ctrl and right-clicking, regardless of the setting
of this option.)
4.11.3 `Shift overrides application's use of mouse'
PuTTY allows the server to send control codes that let it take
over the mouse and use it for purposes other than copy and paste.
Applications which use this feature include the text-mode web
browser `links', the Usenet newsreader `trn' version 4, and the file
manager `mc' (Midnight Commander).
When running one of these applications, pressing the mouse buttons
no longer performs copy and paste. If you do need to copy and paste,
you can still do so if you hold down Shift while you do your mouse
clicks.
However, it is possible in theory for applications to even detect
and make use of Shift + mouse clicks. We don't know of any
applications that do this, but in case someone ever writes one,
unchecking the `Shift overrides application's use of mouse' checkbox
will cause Shift + mouse clicks to go to the server as well (so that
mouse-driven copy and paste will be completely disabled).
If you want to prevent the application from taking over the mouse at
all, you can do this using the Features control panel; see section
4.6.2.
4.11.4 Default selection mode
As described in section 3.1.1, PuTTY has two modes of selecting
text to be copied to the clipboard. In the default mode (`Normal'),
dragging the mouse from point A to point B selects to the end of
the line containing A, all the lines in between, and from the very
beginning of the line containing B. In the other mode (`Rectangular
block'), dragging the mouse between two points defines a rectangle,
and everything within that rectangle is copied.
Normally, you have to hold down Alt while dragging the mouse to
select a rectangular block. Using the `Default selection mode'
control, you can set rectangular selection as the default, and then
you have to hold down Alt to get the _normal_ behaviour.
4.11.5 Configuring word-by-word selection
PuTTY will select a word at a time in the terminal window if you
double-click to begin the drag. This panel allows you to control
precisely what is considered to be a word.
Each character is given a _class_, which is a small number
(typically 0, 1 or 2). PuTTY considers a single word to be any
number of adjacent characters in the same class. So by modifying the
assignment of characters to classes, you can modify the word-by-word
selection behaviour.
In the default configuration, the character classes are:
- Class 0 contains white space and control characters.
- Class 1 contains most punctuation.
- Class 2 contains letters, numbers and a few pieces of
punctuation (the double quote, minus sign, period, forward slash
and underscore).
So, for example, if you assign the `@' symbol into character class
2, you will be able to select an e-mail address with just a double
click.
In order to adjust these assignments, you start by selecting a group
of characters in the list box. Then enter a class number in the edit
box below, and press the `Set' button.
This mechanism currently only covers ASCII characters, because it
isn't feasible to expand the list to cover the whole of Unicode.
Character class definitions can be modified by control sequences
sent by the server. This configuration option controls the _default_
state, which will be restored when you reset the terminal (see
section 3.1.3.6). However, if you modify this option in mid-session
using `Change Settings', it will take effect immediately.
4.12 The Colours panel
The Colours panel allows you to control PuTTY's use of colour.
4.12.1 `Allow terminal to specify ANSI colours'
This option is enabled by default. If it is disabled, PuTTY will
ignore any control sequences sent by the server to request coloured
text.
If you have a particularly garish application, you might want to
turn this option off and make PuTTY only use the default foreground
and background colours.
4.12.2 `Allow terminal to use xterm 256-colour mode'
This option is enabled by default. If it is disabled, PuTTY will
ignore any control sequences sent by the server which use the
extended 256-colour mode supported by recent versions of xterm.
If you have an application which is supposed to use 256-colour mode
and it isn't working, you may find you need to tell your server that
your terminal supports 256 colours. On Unix, you do this by ensuring
that the setting of TERM describes a 256-colour-capable terminal.
You can check this using a command such as `infocmp':
$ infocmp | grep colors
colors#256, cols#80, it#8, lines#24, pairs#256,
If you do not see `colors#256' in the output, you may need to change
your terminal setting. On modern Linux machines, you could try
`xterm-256color'.
4.12.3 `Bolded text is a different colour'
When the server sends a control sequence indicating that some text
should be displayed in bold, PuTTY can handle this two ways. It can
either change the font for a bold version, or use the same font in a
brighter colour. This control lets you choose which.
By default the box is checked, so non-bold text is displayed in
light grey and bold text is displayed in bright white (and similarly
in other colours). If you uncheck the box, bold and non-bold text
will be displayed in the same colour, and instead the font will
change to indicate the difference.
4.12.4 `Attempt to use logical palettes'
Logical palettes are a mechanism by which a Windows application
running on an 8-bit colour display can select precisely the colours
it wants instead of going with the Windows standard defaults.
If you are not getting the colours you ask for on an 8-bit display,
you can try enabling this option. However, be warned that it's never
worked very well.
4.12.5 `Use system colours'
Enabling this option will cause PuTTY to ignore the configured
colours for `Default Background/Foreground' and `Cursor Colour/Text'
(see section 4.12.6), instead going with the system-wide defaults.
Note that non-bold and bold text will be the same colour if this
option is enabled. You might want to change to indicating bold text
by font changes (see section 4.12.3).
4.12.6 Adjusting the colours in the terminal window
The main colour control allows you to specify exactly what colours
things should be displayed in. To modify one of the PuTTY colours,
use the list box to select which colour you want to modify. The
RGB values for that colour will appear on the right-hand side of
the list box. Now, if you press the `Modify' button, you will be
presented with a colour selector, in which you can choose a new
colour to go in place of the old one.
PuTTY allows you to set the cursor colour, the default foreground
and background, and the precise shades of all the ANSI configurable
colours (black, red, green, yellow, blue, magenta, cyan, and white).
You can also modify the precise shades used for the bold versions
of these colours; these are used to display bold text if you have
selected `Bolded text is a different colour', and can also be used
if the server asks specifically to use them. (Note that `Default
Bold Background' is _not_ the background colour used for bold
text; it is only used if the server specifically asks for a bold
background.)
4.13 The Connection panel
The Connection panel allows you to configure options that apply to
more than one type of connection.
4.13.1 Using keepalives to prevent disconnection
If you find your sessions are closing unexpectedly (most often with
`Connection reset by peer') after they have been idle for a while,
you might want to try using this option.
Some network routers and firewalls need to keep track of all
connections through them. Usually, these firewalls will assume a
connection is dead if no data is transferred in either direction
after a certain time interval. This can cause PuTTY sessions to be
unexpectedly closed by the firewall if no traffic is seen in the
session for some time.
The keepalive option (`Seconds between keepalives') allows you
to configure PuTTY to send data through the session at regular
intervals, in a way that does not disrupt the actual terminal
session. If you find your firewall is cutting idle connections off,
you can try entering a non-zero value in this field. The value
is measured in seconds; so, for example, if your firewall cuts
connections off after ten minutes then you might want to enter 300
seconds (5 minutes) in the box.
Note that keepalives are not always helpful. They help if you have
a firewall which drops your connection after an idle period; but
if the network between you and the server suffers from breaks in
connectivity then keepalives can actually make things worse. If a
session is idle, and connectivity is temporarily lost between the
endpoints, but the connectivity is restored before either side
tries to send anything, then there will be no problem - neither
endpoint will notice that anything was wrong. However, if one side
does send something during the break, it will repeatedly try to
re-send, and eventually give up and abandon the connection. Then
when connectivity is restored, the other side will find that the
first side doesn't believe there is an open connection any more.
Keepalives can make this sort of problem worse, because they
increase the probability that PuTTY will attempt to send data
during a break in connectivity. Therefore, you might find they help
connection loss, or you might find they make it worse, depending on
what _kind_ of network problems you have between you and the server.
Keepalives are only supported in Telnet and SSH; the Rlogin and Raw
protocols offer no way of implementing them. (For an alternative,
see section 4.13.3.)
Note that if you are using SSH-1 and the server has a bug that makes
it unable to deal with SSH-1 ignore messages (see section 4.23.1),
enabling keepalives will have no effect.
4.13.2 `Disable Nagle's algorithm'
Nagle's algorithm is a detail of TCP/IP implementations that tries
to minimise the number of small data packets sent down a network
connection. With Nagle's algorithm enabled, PuTTY's bandwidth usage
will be slightly more efficient; with it disabled, you may find you
get a faster response to your keystrokes when connecting to some
types of server.
The Nagle algorithm is disabled by default for interactive
connections.
4.13.3 `Enable TCP keepalives'
_NOTE:_ TCP keepalives should not be confused with the application-
level keepalives described in section 4.13.1. If in doubt, you
probably want application-level keepalives; TCP keepalives are
provided for completeness.
The idea of TCP keepalives is similar to application-level
keepalives, and the same caveats apply. The main differences are:
- TCP keepalives are available on _all_ connection types,
including Raw and Rlogin.
- The interval between TCP keepalives is usually much longer,
typically two hours; this is set by the operating system, and
cannot be configured within PuTTY.
- If the operating system does not receive a response to a
keepalive, it may send out more in quick succession and
terminate the connection if no response is received.
TCP keepalives may be more useful for ensuring that half-open
connections are terminated than for keeping a connection alive.
TCP keepalives are disabled by default.
4.13.4 `Internet protocol'
This option allows the user to select between the old and new
Internet protocols and addressing schemes (IPv4 and IPv6). The
default setting is `Auto', which means PuTTY will do something
sensible and try to guess which protocol you wanted. (If you specify
a literal Internet address, it will use whichever protocol that
address implies. If you provide a hostname, it will see what kinds
of address exist for that hostname; it will use IPv6 if there is an
IPv6 address available, and fall back to IPv4 if not.)
If you need to force PuTTY to use a particular protocol, you can
explicitly set this to `IPv4' or `IPv6'.
4.14 The Data panel
The Data panel allows you to configure various pieces of data which
can be sent to the server to affect your connection at the far end.
Each option on this panel applies to more than one protocol.
Options which apply to only one protocol appear on that protocol's
configuration panels.
4.14.1 `Auto-login username'
All three of the SSH, Telnet and Rlogin protocols allow you to
specify what user name you want to log in as, without having to type
it explicitly every time. (Some Telnet servers don't support this.)
In this box you can type that user name.
4.14.2 `Terminal-type string'
Most servers you might connect to with PuTTY are designed to be
connected to from lots of different types of terminal. In order to
send the right control sequences to each one, the server will need
to know what type of terminal it is dealing with. Therefore, each of
the SSH, Telnet and Rlogin protocols allow a text string to be sent
down the connection describing the terminal.
PuTTY attempts to emulate the Unix `xterm' program, and by default
it reflects this by sending `xterm' as a terminal-type string. If
you find this is not doing what you want - perhaps the remote system
reports `Unknown terminal type' - you could try setting this to
something different, such as `vt220'.
If you're not sure whether a problem is due to the terminal type
setting or not, you probably need to consult the manual for your
application or your server.
4.14.3 `Terminal speeds'
The Telnet, Rlogin, and SSH protocols allow the client to specify
terminal speeds to the server.
This parameter does _not_ affect the actual speed of the connection,
which is always `as fast as possible'; it is just a hint that is
sometimes used by server software to modify its behaviour. For
instance, if a slow speed is indicated, the server may switch to a
less bandwidth-hungry display mode.
The value is usually meaningless in a network environment, but PuTTY
lets you configure it, in case you find the server is reacting badly
to the default value.
The format is a pair of numbers separated by a comma, for instance,
`38400,38400'. The first number represents the output speed (_from_
the server) in bits per second, and the second is the input speed
(_to_ the server). (Only the first is used in the Rlogin protocol.)
This option has no effect on Raw connections.
4.14.4 Setting environment variables on the server
The Telnet protocol provides a means for the client to pass
environment variables to the server. Many Telnet servers have
stopped supporting this feature due to security flaws, but PuTTY
still supports it for the benefit of any servers which have found
other ways around the security problems than just disabling the
whole mechanism.
Version 2 of the SSH protocol also provides a similar mechanism,
which is easier to implement without security flaws. Newer SSH-2
servers are more likely to support it than older ones.
This configuration data is not used in the SSH-1, rlogin or raw
protocols.
To add an environment variable to the list transmitted down the
connection, you enter the variable name in the `Variable' box, enter
its value in the `Value' box, and press the `Add' button. To remove
one from the list, select it in the list box and press `Remove'.
4.15 The Proxy panel
The Proxy panel allows you to configure PuTTY to use various types
of proxy in order to make its network connections. The settings in
this panel affect the primary network connection forming your PuTTY
session, but also any extra connections made as a result of SSH port
forwarding (see section 3.5).
4.15.1 Setting the proxy type
The `Proxy type' radio buttons allow you to configure what type of
proxy you want PuTTY to use for its network connections. The default
setting is `None'; in this mode no proxy is used for any connection.
- Selecting `HTTP' allows you to proxy your connections through a
web server supporting the HTTP CONNECT command, as documented in
RFC 2817.
- Selecting `SOCKS 4' or `SOCKS 5' allows you to proxy your
connections through a SOCKS server.
- Many firewalls implement a less formal type of proxy in which
a user can make a Telnet connection directly to the firewall
machine and enter a command such as `connect myhost.com 22' to
connect through to an external host. Selecting `Telnet' allows
you to tell PuTTY to use this type of proxy.
4.15.2 Excluding parts of the network from proxying
Typically you will only need to use a proxy to connect to non-local
parts of your network; for example, your proxy might be required for
connections outside your company's internal network. In the `Exclude
Hosts/IPs' box you can enter ranges of IP addresses, or ranges of
DNS names, for which PuTTY will avoid using the proxy and make a
direct connection instead.
The `Exclude Hosts/IPs' box may contain more than one exclusion
range, separated by commas. Each range can be an IP address or a DNS
name, with a `*' character allowing wildcards. For example:
*.example.com
This excludes any host with a name ending in `.example.com' from
proxying.
192.168.88.*
This excludes any host with an IP address starting with 192.168.88
from proxying.
192.168.88.*,*.example.com
This excludes both of the above ranges at once.
Connections to the local host (the host name `localhost', and any
loopback IP address) are never proxied, even if the proxy exclude
list does not explicitly contain them. It is very unlikely that this
behaviour would ever cause problems, but if it does you can change
it by enabling `Consider proxying local host connections'.
Note that if you are doing DNS at the proxy (see section 4.15.3),
you should make sure that your proxy exclusion settings do not
depend on knowing the IP address of a host. If the name is passed on
to the proxy without PuTTY looking it up, it will never know the IP
address and cannot check it against your list.
4.15.3 Name resolution when using a proxy
If you are using a proxy to access a private network, it can make a
difference whether DNS name resolution is performed by PuTTY itself
(on the client machine) or performed by the proxy.
The `Do DNS name lookup at proxy end' configuration option allows
you to control this. If you set it to `No', PuTTY will always do its
own DNS, and will always pass an IP address to the proxy. If you set
it to `Yes', PuTTY will always pass host names straight to the proxy
without trying to look them up first.
If you set this option to `Auto' (the default), PuTTY will do
something it considers appropriate for each type of proxy. Telnet
and HTTP proxies will have host names passed straight to them; SOCKS
proxies will not.
Note that if you are doing DNS at the proxy, you should make sure
that your proxy exclusion settings (see section 4.15.2) do not
depend on knowing the IP address of a host. If the name is passed on
to the proxy without PuTTY looking it up, it will never know the IP
address and cannot check it against your list.
The original SOCKS 4 protocol does not support proxy-side DNS. There
is a protocol extension (SOCKS 4A) which does support it, but not
all SOCKS 4 servers provide this extension. If you enable proxy DNS
and your SOCKS 4 server cannot deal with it, this might be why.
4.15.4 Username and password
If your proxy requires authentication, you can enter a username and
a password in the `Username' and `Password' boxes.
Note that if you save your session, the proxy password will be saved
in plain text, so anyone who can access your PuTTY configuration
data will be able to discover it.
Authentication is not fully supported for all forms of proxy:
- Username and password authentication is supported for HTTP
proxies and SOCKS 5 proxies.
- With SOCKS 5, authentication is via CHAP if the proxy
supports it (this is not supported in PuTTYtel); otherwise
the password is sent to the proxy in plain text.
- With HTTP proxying, the only currently supported
authentication method is `basic', where the password is sent
to the proxy in plain text.
- SOCKS 4 can use the `Username' field, but does not support
passwords.
- You can specify a way to include a username and password in the
Telnet proxy command (see section 4.15.5).
4.15.5 Specifying the Telnet proxy command
If you are using the Telnet proxy type, the usual command required
by the firewall's Telnet server is `connect', followed by a host
name and a port number. If your proxy needs a different command, you
can enter an alternative here.
In this string, you can use `\n' to represent a new-line, `\r' to
represent a carriage return, `\t' to represent a tab character, and
`\x' followed by two hex digits to represent any other character.
`\\' is used to encode the `\' character itself.
Also, the special strings `%host' and `%port' will be replaced by
the host name and port number you want to connect to. The strings
`%user' and `%pass' will be replaced by the proxy username and
password you specify. To get a literal `%' sign, enter `%%'.
If the Telnet proxy server prompts for a username and password
before commands can be sent, you can use a command such as:
%user\n%pass\nconnect %host %port\n
This will send your username and password as the first two lines
to the proxy, followed by a command to connect to the desired host
and port. Note that if you do not include the `%user' or `%pass'
tokens in the Telnet command, then the `Username' and `Password'
configuration fields will be ignored.
4.16 The Telnet panel
The Telnet panel allows you to configure options that only apply to
Telnet sessions.
4.16.1 `Handling of OLD_ENVIRON ambiguity'
The original Telnet mechanism for passing environment variables was
badly specified. At the time the standard (RFC 1408) was written,
BSD telnet implementations were already supporting the feature, and
the intention of the standard was to describe the behaviour the BSD
implementations were already using.
Sadly there was a typing error in the standard when it was issued,
and two vital function codes were specified the wrong way round. BSD
implementations did not change, and the standard was not corrected.
Therefore, it's possible you might find either BSD or RFC-compliant
implementations out there. This switch allows you to choose which
one PuTTY claims to be.
The problem was solved by issuing a second standard, defining a
new Telnet mechanism called NEW_ENVIRON, which behaved exactly
like the original OLD_ENVIRON but was not encumbered by existing
implementations. Most Telnet servers now support this, and it's
unambiguous. This feature should only be needed if you have trouble
passing environment variables to quite an old server.
4.16.2 Passive and active Telnet negotiation modes
In a Telnet connection, there are two types of data passed between
the client and the server: actual text, and _negotiations_ about
which Telnet extra features to use.
PuTTY can use two different strategies for negotiation:
- In _active_ mode, PuTTY starts to send negotiations as soon as
the connection is opened.
- In _passive_ mode, PuTTY will wait to negotiate until it sees a
negotiation from the server.
The obvious disadvantage of passive mode is that if the server is
also operating in a passive mode, then negotiation will never begin
at all. For this reason PuTTY defaults to active mode.
However, sometimes passive mode is required in order to successfully
get through certain types of firewall and Telnet proxy server. If
you have confusing trouble with a firewall, you could try enabling
passive mode to see if it helps.
4.16.3 `Keyboard sends Telnet special commands'
If this box is checked, several key sequences will have their normal
actions modified:
- the Backspace key on the keyboard will send the Telnet special
backspace code;
- Control-C will send the Telnet special Interrupt Process code;
- Control-Z will send the Telnet special Suspend Process code.
You probably shouldn't enable this unless you know what you're
doing.
4.16.4 `Return key sends Telnet New Line instead of ^M'
Unlike most other remote login protocols, the Telnet protocol has
a special `new line' code that is not the same as the usual line
endings of Control-M or Control-J. By default, PuTTY sends the
Telnet New Line code when you press Return, instead of sending
Control-M as it does in most other protocols.
Most Unix-style Telnet servers don't mind whether they receive
Telnet New Line or Control-M; some servers do expect New Line,
and some servers prefer to see ^M. If you are seeing surprising
behaviour when you press Return in a Telnet session, you might try
turning this option off to see if it helps.
4.17 The Rlogin panel
The Rlogin panel allows you to configure options that only apply to
Rlogin sessions.
4.17.1 `Local username'
Rlogin allows an automated (password-free) form of login by means
of a file called `.rhosts' on the server. You put a line in your
`.rhosts' file saying something like `jbloggs@pc1.example.com', and
then when you make an Rlogin connection the client transmits the
username of the user running the Rlogin client. The server checks
the username and hostname against `.rhosts', and if they match it
does not ask for a password.
This only works because Unix systems contain a safeguard to stop a
user from pretending to be another user in an Rlogin connection.
Rlogin connections have to come from port numbers below 1024, and
Unix systems prohibit this to unprivileged processes; so when the
server sees a connection from a low-numbered port, it assumes the
client end of the connection is held by a privileged (and therefore
trusted) process, so it believes the claim of who the user is.
Windows does not have this restriction: _any_ user can initiate
an outgoing connection from a low-numbered port. Hence, the
Rlogin `.rhosts' mechanism is completely useless for securely
distinguishing several different users on a Windows machine. If you
have a `.rhosts' entry pointing at a Windows PC, you should assume
that _anyone_ using that PC can spoof your username in an Rlogin
connection and access your account on the server.
The `Local username' control allows you to specify what user name
PuTTY should claim you have, in case it doesn't match your Windows
user name (or in case you didn't bother to set up a Windows user
name).
4.18 The SSH panel
The SSH panel allows you to configure options that only apply to SSH
sessions.
4.18.1 Executing a specific command on the server
In SSH, you don't have to run a general shell session on the server.
Instead, you can choose to run a single specific command (such as
a mail user agent, for example). If you want to do this, enter the
command in the `Remote command' box.
4.18.2 `Don't allocate a pseudo-terminal'
When connecting to a Unix system, most interactive shell sessions
are run in a _pseudo-terminal_, which allows the Unix system to
pretend it's talking to a real physical terminal device but allows
the SSH server to catch all the data coming from that fake device
and send it back to the client.
Occasionally you might find you have a need to run a session _not_
in a pseudo-terminal. In PuTTY, this is generally only useful for
very specialist purposes; although in Plink (see chapter 7) it is
the usual way of working.
4.18.3 `Don't start a shell or command at all'
If you tick this box, PuTTY will not attempt to run a shell or
command after connecting to the remote server. You might want to
use this option if you are only using the SSH connection for port
forwarding, and your user account on the server does not have the
ability to run a shell.
This feature is only available in SSH protocol version 2 (since the
version 1 protocol assumes you will always want to run a shell).
This feature can also be enabled using the `-N' command-line option;
see section 3.7.3.12.
If you use this feature in Plink, you will not be able to terminate
the Plink process by any graceful means; the only way to kill it
will be by pressing Control-C or sending a kill signal from another
program.
4.18.4 `Enable compression'
This enables data compression in the SSH connection: data sent by
the server is compressed before sending, and decompressed at the
client end. Likewise, data sent by PuTTY to the server is compressed
first and the server decompresses it at the other end. This can help
make the most of a low-bandwidth connection.
4.18.5 `Preferred SSH protocol version'
This allows you to select whether you would like to use SSH protocol
version 1 or version 2.
PuTTY will attempt to use protocol 1 if the server you connect to
does not offer protocol 2, and vice versa.
If you select `1 only' or `2 only' here, PuTTY will only connect if
the server you connect to offers the SSH protocol version you have
specified.
4.18.6 Encryption algorithm selection
PuTTY supports a variety of different encryption algorithms, and
allows you to choose which one you prefer to use. You can do this by
dragging the algorithms up and down in the list box (or moving them
using the Up and Down buttons) to specify a preference order. When
you make an SSH connection, PuTTY will search down the list from the
top until it finds an algorithm supported by the server, and then
use that.
PuTTY currently supports the following algorithms:
- AES (Rijndael) - 256, 192, or 128-bit CBC (SSH-2 only)
- Blowfish - 128-bit CBC
- Triple-DES - 168-bit CBC
- Single-DES - 56-bit CBC (see below for SSH-2)
If the algorithm PuTTY finds is below the `warn below here' line,
you will see a warning box when you make the connection:
The first cipher supported by the server
is single-DES, which is below the configured
warning threshold.
Do you want to continue with this connection?
This warns you that the first available encryption is not a very
secure one. Typically you would put the `warn below here' line
between the encryptions you consider secure and the ones you
consider substandard. By default, PuTTY supplies a preference order
intended to reflect a reasonable preference in terms of security and
speed.
In SSH-2, the encryption algorithm is negotiated independently for
each direction of the connection, although PuTTY does not support
separate configuration of the preference orders. As a result you may
get two warnings similar to the one above, possibly with different
encryptions.
Single-DES is not recommended in the SSH-2 draft protocol standards,
but one or two server implementations do support it. PuTTY can use
single-DES to interoperate with these servers if you enable the
`Enable legacy use of single-DES in SSH-2' option; by default this
is disabled and PuTTY will stick to recommended ciphers.
4.19 The Kex panel
The Kex panel (short for `key exchange') allows you to configure
options related to SSH-2 key exchange.
Key exchange occurs at the start of an SSH connection (and
occasionally thereafter); it establishes a shared secret that
is used as the basis for all of SSH's security features. It is
therefore very important for the security of the connection that the
key exchange is secure.
Key exchange is a cryptographically intensive process; if either
the client or the server is a relatively slow machine, the slower
methods may take several tens of seconds to complete.
If connection startup is too slow, or the connection hangs
periodically, you may want to try changing these settings.
If you don't understand what any of this means, it's safe to leave
these settings alone.
This entire panel is only relevant to SSH protocol version 2; none
of these settings affect SSH-1 at all.
4.19.1 Key exchange algorithm selection
PuTTY supports a variety of SSH-2 key exchange methods, and allows
you to choose which one you prefer to use; configuration is similar
to cipher selection (see section 4.18.6).
PuTTY currently supports the following varieties of Diffie-Hellman
key exchange:
- `Group 14': a well-known 2048-bit group.
- `Group 1': a well-known 1024-bit group. This is less secure than
group 14, but may be faster with slow client or server machines,
and may be the only method supported by older server software.
- `Group exchange': with this method, instead of using a fixed
group, PuTTY requests that the server suggest a group to use for
key exchange; the server can avoid groups known to be weak, and
possibly invent new ones over time, without any changes required
to PuTTY's configuration. We recommend use of this method, if
possible.
If the first algorithm PuTTY finds is below the `warn below here'
line, you will see a warning box when you make the connection,
similar to that for cipher selection (see section 4.18.6).
4.19.2 Repeat key exchange
If the session key negotiated at connection startup is used too much
or for too long, it may become feasible to mount attacks against the
SSH connection. Therefore, the SSH-2 protocol specifies that a new
key exchange should take place every so often; this can be initiated
by either the client or the server.
While this renegotiation is taking place, no data can pass through
the SSH connection, so it may appear to `freeze'. (The occurrence
of repeat key exchange is noted in the Event Log; see section
3.1.3.1.) Usually the same algorithm is used as at the start of the
connection, with a similar overhead.
These options control how often PuTTY will initiate a repeat key
exchange (`rekey'). You can also force a key exchange at any time
from the Special Commands menu (see section 3.1.3.2).
- `Max minutes before rekey' specifies the amount of time that
is allowed to elapse before a rekey is initiated. If this is
set to zero, PuTTY will not rekey due to elapsed time. The SSH-
2 protocol specification recommends a timeout of at most 60
minutes.
You might have a need to disable time-based rekeys completely
for the same reasons that keepalives aren't always helpful. If
you anticipate suffering a network dropout of several hours in
the middle of an SSH connection, but were not actually planning
to send _data_ down that connection during those hours, then an
attempted rekey in the middle of the dropout will probably cause
the connection to be abandoned, whereas if rekeys are disabled
then the connection should in principle survive (in the absence
of interfering firewalls). See section 4.13.1 for more discussion
of these issues; for these purposes, rekeys have much the same
properties as keepalives. (Except that rekeys have cryptographic
value in themselves, so you should bear that in mind when deciding
whether to turn them off.) Note, however, the the SSH _server_ can
still initiate rekeys.
- `Max data before rekey' specifies the amount of data (in bytes)
that is permitted to flow in either direction before a rekey is
initiated. If this is set to zero, PuTTY will not rekey due to
transferred data. The SSH-2 protocol specification recommends a
limit of at most 1 gigabyte.
As well as specifying a value in bytes, the following shorthand
can be used:
- `1k' specifies 1 kilobyte (1024 bytes).
- `1M' specifies 1 megabyte (1024 kilobytes).
- `1G' specifies 1 gigabyte (1024 megabytes).
Disabling data-based rekeys entirely is a bad idea. The integrity,
and to a lesser extent, confidentiality of the SSH-2 protocol depend
in part on rekeys occuring before a 32-bit packet sequence number
wraps around. Unlike time-based rekeys, data-based rekeys won't
occur when the SSH connection is idle, so they shouldn't cause the
same problems. The SSH-1 protocol, incidentally, has even weaker
integrity protection than SSH-2 without rekeys.
4.20 The Auth panel
The Auth panel allows you to configure authentication options for
SSH sessions.
4.20.1 `Attempt TIS or CryptoCard authentication'
TIS and CryptoCard authentication are simple challenge/response
forms of authentication available in SSH protocol version 1 only.
You might use them if you were using S/Key one-time passwords, for
example, or if you had a physical security token that generated
responses to authentication challenges.
With this switch enabled, PuTTY will attempt these forms of
authentication if the server is willing to try them. You will
be presented with a challenge string (which will be different
every time) and must supply the correct response in order to log
in. If your server supports this, you should talk to your system
administrator about precisely what form these challenges and
responses take.
4.20.2 `Attempt keyboard-interactive authentication'
The SSH-2 equivalent of TIS authentication is called `keyboard-
interactive'. It is a flexible authentication method using an
arbitrary sequence of requests and responses; so it is not only
useful for challenge/response mechanisms such as S/Key, but it can
also be used for (for example) asking the user for a new password
when the old one has expired.
PuTTY leaves this option enabled by default, but supplies a switch
to turn it off in case you should have trouble with it.
4.20.3 `Allow agent forwarding'
This option allows the SSH server to open forwarded connections back
to your local copy of Pageant. If you are not running Pageant, this
option will do nothing.
See chapter 9 for general information on Pageant, and section 9.4
for information on agent forwarding. Note that there is a security
risk involved with enabling this option; see section 9.5 for
details.
4.20.4 `Allow attempted changes of username in SSH-2'
In the SSH-1 protocol, it is impossible to change username after
failing to authenticate. So if you mis-type your username at the
PuTTY `login as:' prompt, you will not be able to change it except
by restarting PuTTY.
The SSH-2 protocol _does_ allow changes of username, in principle,
but does not make it mandatory for SSH-2 servers to accept them.
In particular, OpenSSH does not accept a change of username; once
you have sent one username, it will reject attempts to try to
authenticate as another user. (Depending on the version of OpenSSH,
it may quietly return failure for all login attempts, or it may send
an error message.)
For this reason, PuTTY will by default not prompt you for your
username more than once, in case the server complains. If you know
your server can cope with it, you can enable the `Allow attempted
changes of username' option to modify PuTTY's behaviour.
4.20.5 `Private key file for authentication'
This box is where you enter the name of your private key file if you
are using public key authentication. See chapter 8 for information
about public key authentication in SSH.
This key must be in PuTTY's native format (`*.'`PPK'). If you have a
private key in another format that you want to use with PuTTY, see
section 8.2.12.
4.21 The X11 panel
The X11 panel allows you to configure forwarding of X11 over an SSH
connection.
If your server lets you run X Window System applications, X11
forwarding allows you to securely give those applications access to
a local X display on your PC.
To enable X11 forwarding, check the `Enable X11 forwarding' box.
If your X display is somewhere unusual, you will need to enter its
location in the `X display location' box; if this is left blank,
PuTTY will try to find a sensible default in the environment, or use
the primary local display (`:0') if that fails.
See section 3.4 for more information about X11 forwarding.
4.21.1 Remote X11 authentication
If you are using X11 forwarding, the virtual X server created on the
SSH server machine will be protected by authorisation data. This
data is invented, and checked, by PuTTY.
The usual authorisation method used for this is called MIT-MAGIC-
COOKIE-1. This is a simple password-style protocol: the X client
sends some cookie data to the server, and the server checks that it
matches the real cookie. The cookie data is sent over an unencrypted
X11 connection; so if you allow a client on a third machine to
access the virtual X server, then the cookie will be sent in the
clear.
PuTTY offers the alternative protocol XDM-AUTHORIZATION-1. This is
a cryptographically authenticated protocol: the data sent by the X
client is different every time, and it depends on the IP address
and port of the client's end of the connection and is also stamped
with the current time. So an eavesdropper who captures an XDM-
AUTHORIZATION-1 string cannot immediately re-use it for their own X
connection.
PuTTY's support for XDM-AUTHORIZATION-1 is a somewhat experimental
feature, and may encounter several problems:
- Some X clients probably do not even support XDM-AUTHORIZATION-
1, so they will not know what to do with the data PuTTY has
provided.
- This authentication mechanism will only work in SSH-2. In SSH-
1, the SSH server does not tell the client the source address
of a forwarded connection in a machine-readable format, so it's
impossible to verify the XDM-AUTHORIZATION-1 data.
- You may find this feature causes problems with some SSH servers,
which will not clean up XDM-AUTHORIZATION-1 data after a
session, so that if you then connect to the same server using
a client which only does MIT-MAGIC-COOKIE-1 and are allocated
the same remote display number, you might find that out-of-date
authentication data is still present on your server and your X
connections fail.
PuTTY's default is MIT-MAGIC-COOKIE-1. If you change it, you should
be sure you know what you're doing.
4.22 The Tunnels panel
The Tunnels panel allows you to configure tunnelling of arbitrary
connection types through an SSH connection.
Port forwarding allows you to tunnel other types of network
connection down an SSH session. See section 3.5 for a general
discussion of port forwarding and how it works.
The port forwarding section in the Tunnels panel shows a list of all
the port forwardings that PuTTY will try to set up when it connects
to the server. By default no port forwardings are set up, so this
list is empty.
To add a port forwarding:
- Set one of the `Local' or `Remote' radio buttons, depending on
whether you want to forward a local port to a remote destination
(`Local') or forward a remote port to a local destination
(`Remote'). Alternatively, select `Dynamic' if you want PuTTY to
provide a local SOCKS 4/4A/5 proxy on a local port.
- Enter a source port number into the `Source port' box. For local
forwardings, PuTTY will listen on this port of your PC. For
remote forwardings, your SSH server will listen on this port of
the remote machine. Note that most servers will not allow you to
listen on port numbers less than 1024.
- If you have selected `Local' or `Remote' (this step is not
needed with `Dynamic'), enter a hostname and port number
separated by a colon, in the `Destination' box. Connections
received on the source port will be directed to this
destination. For example, to connect to a POP-3 server, you
might enter `popserver.example.com:110'.
- Click the `Add' button. Your forwarding details should appear in
the list box.
To remove a port forwarding, simply select its details in the list
box, and click the `Remove' button.
In the `Source port' box, you can also optionally enter an IP
address to listen on, by specifying (for instance) `127.0.0.5:79'.
See section 3.5 for more information on how this works and its
restrictions.
In place of port numbers, you can enter service names, if they are
known to the local system. For instance, in the `Destination' box,
you could enter `popserver.example.com:pop3'.
You can modify the currently active set of port forwardings in mid-
session using `Change Settings' (see section 3.1.3.4). If you delete
a local or dynamic port forwarding in mid-session, PuTTY will stop
listening for connections on that port, so it can be re-used by
another program. If you delete a remote port forwarding, note that:
- The SSH-1 protocol contains no mechanism for asking the server
to stop listening on a remote port.
- The SSH-2 protocol does contain such a mechanism, but not all
SSH servers support it. (In particular, OpenSSH does not support
it in any version earlier than 3.9.)
If you ask to delete a remote port forwarding and PuTTY cannot make
the server actually stop listening on the port, it will instead
just start refusing incoming connections on that port. Therefore,
although the port cannot be reused by another program, you can at
least be reasonably sure that server-side programs can no longer
access the service at your end of the port forwarding.
If you delete a forwarding, any existing connections established
using that forwarding remain open. Similarly, changes to global
settings such as `Local ports accept connections from other hosts'
only take effect on new forwardings.
4.22.1 Controlling the visibility of forwarded ports
The source port for a forwarded connection usually does not accept
connections from any machine except the SSH client or server machine
itself (for local and remote forwardings respectively). There are
controls in the Tunnels panel to change this:
- The `Local ports accept connections from other hosts' option
allows you to set up local-to-remote port forwardings in such a
way that machines other than your client PC can connect to the
forwarded port. (This also applies to dynamic SOCKS forwarding.)
- The `Remote ports do the same' option does the same thing for
remote-to-local port forwardings (so that machines other than
the SSH server machine can connect to the forwarded port.) Note
that this feature is only available in the SSH-2 protocol, and
not all SSH-2 servers support it (OpenSSH 3.0 does not, for
example).
4.22.2 Selecting Internet protocol version for forwarded ports
This switch allows you to select a specific Internet protocol (IPv4
or IPv6) for the local end of a forwarded port. By default, it is
set on `Auto', which means that:
- for a local-to-remote port forwarding, PuTTY will listen for
incoming connections in both IPv4 and (if available) IPv6
- for a remote-to-local port forwarding, PuTTY will choose a
sensible protocol for the outgoing connection.
Note that some operating systems may listen for incoming connections
in IPv4 even if you specifically asked for IPv6, because their IPv4
and IPv6 protocol stacks are linked together. Apparently Linux does
this, and Windows does not. So if you're running PuTTY on Windows
and you tick `IPv6' for a local or dynamic port forwarding, it will
_only_ be usable by connecting to it using IPv6; whereas if you do
the same on Linux, you can also use it with IPv4. However, ticking
`Auto' should always give you a port which you can connect to using
either protocol.
4.23 The Bugs panel
Not all SSH servers work properly. Various existing servers have
bugs in them, which can make it impossible for a client to talk to
them unless it knows about the bug and works around it.
Since most servers announce their software version number at the
beginning of the SSH connection, PuTTY will attempt to detect which
bugs it can expect to see in the server and automatically enable
workarounds. However, sometimes it will make mistakes; if the server
has been deliberately configured to conceal its version number, or
if the server is a version which PuTTY's bug database does not know
about, then PuTTY will not know what bugs to expect.
The Bugs panel allows you to manually configure the bugs PuTTY
expects to see in the server. Each bug can be configured in three
states:
- `Off': PuTTY will assume the server does not have the bug.
- `On': PuTTY will assume the server _does_ have the bug.
- `Auto': PuTTY will use the server's version number announcement
to try to guess whether or not the server has the bug.
4.23.1 `Chokes on SSH-1 ignore messages'
An ignore message (SSH_MSG_IGNORE) is a message in the SSH protocol
which can be sent from the client to the server, or from the server
to the client, at any time. Either side is required to ignore the
message whenever it receives it. PuTTY uses ignore messages to hide
the password packet in SSH-1, so that a listener cannot tell the
length of the user's password; it also uses ignore messages for
connection keepalives (see section 4.13.1).
If this bug is detected, PuTTY will stop using ignore messages.
This means that keepalives will stop working, and PuTTY will
have to fall back to a secondary defence against SSH-1 password-
length eavesdropping. See section 4.23.2. If this bug is enabled
when talking to a correct server, the session will succeed, but
keepalives will not work and the session might be more vulnerable to
eavesdroppers than it could be.
This is an SSH-1-specific bug. No known SSH-2 server fails to deal
with SSH-2 ignore messages.
4.23.2 `Refuses all SSH-1 password camouflage'
When talking to an SSH-1 server which cannot deal with ignore
messages (see section 4.23.1), PuTTY will attempt to disguise the
length of the user's password by sending additional padding _within_
the password packet. This is technically a violation of the SSH-
1 specification, and so PuTTY will only do it when it cannot use
standards-compliant ignore messages as camouflage. In this sense,
for a server to refuse to accept a padded password packet is not
really a bug, but it does make life inconvenient if the server can
also not handle ignore messages.
If this `bug' is detected, PuTTY will have no choice but to
send the user's password with no form of camouflage, so that an
eavesdropping user will be easily able to find out the exact length
of the password. If this bug is enabled when talking to a correct
server, the session will succeed, but will be more vulnerable to
eavesdroppers than it could be.
This is an SSH-1-specific bug. SSH-2 is secure against this type of
attack.
4.23.3 `Chokes on SSH-1 RSA authentication'
Some SSH-1 servers cannot deal with RSA authentication messages at
all. If Pageant is running and contains any SSH-1 keys, PuTTY will
normally automatically try RSA authentication before falling back
to passwords, so these servers will crash when they see the RSA
attempt.
If this bug is detected, PuTTY will go straight to password
authentication. If this bug is enabled when talking to a correct
server, the session will succeed, but of course RSA authentication
will be impossible.
This is an SSH-1-specific bug.
4.23.4 `Miscomputes SSH-2 HMAC keys'
Versions 2.3.0 and below of the SSH server software from ssh.com
compute the keys for their HMAC message authentication codes
incorrectly. A typical symptom of this problem is that PuTTY dies
unexpectedly at the beginning of the session, saying `Incorrect MAC
received on packet'.
If this bug is detected, PuTTY will compute its HMAC keys in the
same way as the buggy server, so that communication will still be
possible. If this bug is enabled when talking to a correct server,
communication will fail.
This is an SSH-2-specific bug.
4.23.5 `Miscomputes SSH-2 encryption keys'
Versions below 2.0.11 of the SSH server software from ssh.com
compute the keys for the session encryption incorrectly. This
problem can cause various error messages, such as `Incoming packet
was garbled on decryption', or possibly even `Out of memory'.
If this bug is detected, PuTTY will compute its encryption keys
in the same way as the buggy server, so that communication will
still be possible. If this bug is enabled when talking to a correct
server, communication will fail.
This is an SSH-2-specific bug.
4.23.6 `Requires padding on SSH-2 RSA signatures'
Versions below 3.3 of OpenSSH require SSH-2 RSA signatures to be
padded with zero bytes to the same length as the RSA key modulus.
The SSH-2 draft specification says that an unpadded signature MUST
be accepted, so this is a bug. A typical symptom of this problem is
that PuTTY mysteriously fails RSA authentication once in every few
hundred attempts, and falls back to passwords.
If this bug is detected, PuTTY will pad its signatures in the way
OpenSSH expects. If this bug is enabled when talking to a correct
server, it is likely that no damage will be done, since correct
servers usually still accept padded signatures because they're used
to talking to OpenSSH.
This is an SSH-2-specific bug.
4.23.7 `Misuses the session ID in SSH-2 PK auth'
Versions below 2.3 of OpenSSH require SSH-2 public-key
authentication to be done slightly differently: the data to be
signed by the client contains the session ID formatted in a
different way. If public-key authentication mysteriously does
not work but the Event Log (see section 3.1.3.1) thinks it has
successfully sent a signature, it might be worth enabling the
workaround for this bug to see if it helps.
If this bug is detected, PuTTY will sign data in the way OpenSSH
expects. If this bug is enabled when talking to a correct server,
SSH-2 public-key authentication will fail.
This is an SSH-2-specific bug.
4.23.8 `Handles SSH-2 key re-exchange badly'
Some SSH servers cannot cope with repeat key exchange at all, and
will ignore attempts by the client to start one. Since PuTTY pauses
the session while performing a repeat key exchange, the effect of
this would be to cause the session to hang after an hour (unless
you have your rekey timeout set differently; see section 4.19.2 for
more about rekeys). Other, very old, SSH servers handle repeat key
exchange even more badly, and disconnect upon receiving a repeat key
exchange request.
If this bug is detected, PuTTY will never initiate a repeat key
exchange. If this bug is enabled when talking to a correct server,
the session should still function, but may be less secure than you
would expect.
This is an SSH-2-specific bug.
4.24 Storing configuration in a file
PuTTY does not currently support storing its configuration in a file
instead of the Registry. However, you can work around this with a
couple of batch files.
You will need a file called (say) `PUTTY.BAT' which imports the
contents of a file into the Registry, then runs PuTTY, exports
the contents of the Registry back into the file, and deletes the
Registry entries. This can all be done using the Regedit command
line options, so it's all automatic. Here is what you need in
`PUTTY.BAT':
@ECHO OFF
regedit /s putty.reg
regedit /s puttyrnd.reg
start /w putty.exe
regedit /ea new.reg HKEY_CURRENT_USER\Software\SimonTatham\PuTTY
copy new.reg putty.reg
del new.reg
regedit /s puttydel.reg
This batch file needs two auxiliary files: `PUTTYRND.REG' which sets
up an initial safe location for the `PUTTY.RND' random seed file,
and `PUTTYDEL.REG' which destroys everything in the Registry once
it's been successfully saved back to the file.
Here is `PUTTYDEL.REG':
REGEDIT4
[-HKEY_CURRENT_USER\Software\SimonTatham\PuTTY]
Here is an example `PUTTYRND.REG' file:
REGEDIT4
[HKEY_CURRENT_USER\Software\SimonTatham\PuTTY]
"RandSeedFile"="a:\\putty.rnd"
You should replace `a:\putty.rnd' with the location where you want
to store your random number data. If the aim is to carry around
PuTTY and its settings on one floppy, you probably want to store it
on the floppy.
Chapter 5: Using PSCP to transfer files securely
------------------------------------------------
PSCP, the PuTTY Secure Copy client, is a tool for transferring files
securely between computers using an SSH connection.
If you have an SSH-2 server, you might prefer PSFTP (see chapter
6) for interactive use. PSFTP does not in general work with SSH-1
servers, however.
5.1 Starting PSCP
PSCP is a command line application. This means that you cannot just
double-click on its icon to run it and instead you have to bring up
a console window. With Windows 95, 98, and ME, this is called an
`MS-DOS Prompt' and with Windows NT, 2000, and XP, it is called a
`Command Prompt'. It should be available from the Programs section
of your Start Menu.
To start PSCP it will need either to be on your `PATH' or in your
current directory. To add the directory containing PSCP to your
`PATH' environment variable, type into the console window:
set PATH=C:\path\to\putty\directory;%PATH%
This will only work for the lifetime of that particular console
window. To set your `PATH' more permanently on Windows NT, 2000, and
XP, use the Environment tab of the System Control Panel. On Windows
95, 98, and ME, you will need to edit your `AUTOEXEC.BAT' to include
a `set' command like the one above.
5.2 PSCP Usage
Once you've got a console window to type into, you can just type
`pscp' on its own to bring up a usage message. This tells you the
version of PSCP you're using, and gives you a brief summary of how
to use PSCP:
Z:\owendadmin>pscp
PuTTY Secure Copy client
Release 0.58
Usage: pscp [options] [user@]host:source target
pscp [options] source [source...] [user@]host:target
pscp [options] -ls [user@]host:filespec
Options:
-V print version information and exit
-pgpfp print PGP key fingerprints and exit
-p preserve file attributes
-q quiet, don't show statistics
-r copy directories recursively
-v show verbose messages
-load sessname Load settings from saved session
-P port connect to specified port
-l user connect with specified username
-pw passw login with specified password
-1 -2 force use of particular SSH protocol version
-4 -6 force use of IPv4 or IPv6
-C enable compression
-i key private key file for authentication
-batch disable all interactive prompts
-unsafe allow server-side wildcards (DANGEROUS)
-sftp force use of SFTP protocol
-scp force use of SCP protocol
(PSCP's interface is much like the Unix `scp' command, if you're
familiar with that.)
5.2.1 The basics
To receive (a) file(s) from a remote server:
pscp [options] [user@]host:source target
So to copy the file `/etc/hosts' from the server `example.com' as
user `fred' to the file `c:\temp\example-hosts.txt', you would type:
pscp fred@example.com:/etc/hosts c:\temp\example-hosts.txt
To send (a) file(s) to a remote server:
pscp [options] source [source...] [user@]host:target
So to copy the local file `c:\documents\foo.txt' to the server
`example.com' as user `fred' to the file `/tmp/foo' you would type:
pscp c:\documents\foo.txt fred@example.com:/tmp/foo
You can use wildcards to transfer multiple files in either
direction, like this:
pscp c:\documents\*.doc fred@example.com:docfiles
pscp fred@example.com:source/*.c c:\source
However, in the second case (using a wildcard for multiple remote
files) you may see a warning saying something like `warning: remote
host tried to write to a file called `terminal.c' when we requested
a file called `*.c'. If this is a wildcard, consider upgrading to
SSH-2 or using the `-unsafe' option. Renaming of this file has been
disallowed'.
This is due to a fundamental insecurity in the old-style SCP
protocol: the client sends the wildcard string (`*.c') to the
server, and the server sends back a sequence of file names that
match the wildcard pattern. However, there is nothing to stop the
server sending back a _different_ pattern and writing over one of
your other files: if you request `*.c', the server might send back
the file name `AUTOEXEC.BAT' and install a virus for you. Since the
wildcard matching rules are decided by the server, the client cannot
reliably verify that the filenames sent back match the pattern.
PSCP will attempt to use the newer SFTP protocol (part of SSH-2)
where possible, which does not suffer from this security flaw. If
you are talking to an SSH-2 server which supports SFTP, you will
never see this warning. (You can force use of the SFTP protocol, if
available, with `-sftp' - see section 5.2.2.6.)
If you really need to use a server-side wildcard with an SSH-1
server, you can use the `-unsafe' command line option with PSCP:
pscp -unsafe fred@example.com:source/*.c c:\source
This will suppress the warning message and the file transfer will
happen. However, you should be aware that by using this option you
are giving the server the ability to write to _any_ file in the
target directory, so you should only use this option if you trust
the server administrator not to be malicious (and not to let the
server machine be cracked by malicious people). Alternatively, do
any such download in a newly created empty directory. (Even in
`unsafe' mode, PSCP will still protect you against the server trying
to get out of that directory using pathnames including `..'.)
5.2.1.1 `user'
The login name on the remote server. If this is omitted, and `host'
is a PuTTY saved session, PSCP will use any username specified by
that saved session. Otherwise, PSCP will attempt to use the local
Windows username.
5.2.1.2 `host'
The name of the remote server, or the name of an existing PuTTY
saved session. In the latter case, the session's settings for
hostname, port number, cipher type and username will be used.
5.2.1.3 `source'
One or more source files. Wildcards are allowed. The syntax of
wildcards depends on the system to which they apply, so if you are
copying _from_ a Windows system _to_ a UNIX system, you should use
Windows wildcard syntax (e.g. `*.*'), but if you are copying _from_
a UNIX system _to_ a Windows system, you would use the wildcard
syntax allowed by your UNIX shell (e.g. `*').
If the source is a remote server and you do not specify a full
pathname (in UNIX, a pathname beginning with a `/' (slash)
character), what you specify as a source will be interpreted
relative to your home directory on the remote server.
5.2.1.4 `target'
The filename or directory to put the file(s). When copying from a
remote server to a local host, you may wish simply to place the
file(s) in the current directory. To do this, you should specify a
target of `.'. For example:
pscp fred@example.com:/home/tom/.emacs .
...would copy `/home/tom/.emacs' on the remote server to the current
directory.
As with the `source' parameter, if the target is on a remote server
and is not a full path name, it is interpreted relative to your home
directory on the remote server.
5.2.2 Options
PSCP accepts all the general command line options supported by the
PuTTY tools, except the ones which make no sense in a file transfer
utility. See section 3.7.3 for a description of these options. (The
ones not supported by PSCP are clearly marked.)
PSCP also supports some of its own options. The following sections
describe PSCP's specific command-line options.
These are the command line options that PSCP accepts.
5.2.2.1 `-ls' list remote files
If the `-ls' option is given, no files are transferred; instead,
remote files are listed. Only a hostname specification and optional
remote file specification need be given. For example:
pscp -ls fred@example.com:dir1
The SCP protocol does not contain within itself a means of listing
files. If SCP is in use, this option therefore assumes that the
server responds appropriately to the command `ls -la'; this may not
work with all servers.
If SFTP is in use, this option should work with all servers.
5.2.2.2 `-p' preserve file attributes
By default, files copied with PSCP are timestamped with the date
and time they were copied. The `-p' option preserves the original
timestamp on copied files.
5.2.2.3 `-q' quiet, don't show statistics
By default, PSCP displays a meter displaying the progress of the
current transfer:
mibs.tar | 168 kB | 84.0 kB/s | ETA: 00:00:13 | 13%
The fields in this display are (from left to right), filename, size
(in kilobytes) of file transferred so far, estimate of how fast the
file is being transferred (in kilobytes per second), estimated time
that the transfer will be complete, and percentage of the file so
far transferred. The `-q' option to PSCP suppresses the printing of
these statistics.
5.2.2.4 `-r' copies directories recursively
By default, PSCP will only copy files. Any directories you specify
to copy will be skipped, as will their contents. The `-r' option
tells PSCP to descend into any directories you specify, and to copy
them and their contents. This allows you to use PSCP to transfer
whole directory structures between machines.
5.2.2.5 `-batch' avoid interactive prompts
If you use the `-batch' option, PSCP will never give an interactive
prompt while establishing the connection. If the server's host key
is invalid, for example (see section 2.2), then the connection will
simply be abandoned instead of asking you what to do next.
This may help PSCP's behaviour when it is used in automated scripts:
using `-batch', if something goes wrong at connection time, the
batch job will fail rather than hang.
5.2.2.6 `-sftp', `-scp' force use of particular protocol
As mentioned in section 5.2.1, there are two different file transfer
protocols in use with SSH. Despite its name, PSCP (like many other
ostensible scp clients) can use either of these protocols.
The older SCP protocol does not have a written specification and
leaves a lot of detail to the server platform. Wildcards are
expanded on the server. The simple design means that any wildcard
specification supported by the server platform (such as brace
expansion) can be used, but also leads to interoperability issues
such as with filename quoting (for instance, where filenames contain
spaces), and also the security issue described in section 5.2.1.
The newer SFTP protocol, which is usually associated with SSH-
2 servers, is specified in a more platform independent way, and
leaves issues such as wildcard syntax up to the client. (PuTTY's
SFTP wildcard syntax is described in section 6.2.2.) This makes it
more consistent across platforms, more suitable for scripting and
automation, and avoids security issues with wildcard matching.
Normally PSCP will attempt to use the SFTP protocol, and only fall
back to the SCP protocol if SFTP is not available on the server.
The `-scp' option forces PSCP to use the SCP protocol or quit.
The `-sftp' option forces PSCP to use the SFTP protocol or quit.
When this option is specified, PSCP looks harder for an SFTP server,
which may allow use of SFTP with SSH-1 depending on server setup.
5.2.3 Return value
PSCP returns an ERRORLEVEL of zero (success) only if the files were
correctly transferred. You can test for this in a batch file, using
code such as this:
pscp file*.* user@hostname:
if errorlevel 1 echo There was an error
5.2.4 Using public key authentication with PSCP
Like PuTTY, PSCP can authenticate using a public key instead of a
password. There are three ways you can do this.
Firstly, PSCP can use PuTTY saved sessions in place of hostnames
(see section 5.2.1.2). So you would do this:
- Run PuTTY, and create a PuTTY saved session (see section 4.1.2)
which specifies your private key file (see section 4.20.5). You
will probably also want to specify a username to log in as (see
section 4.14.1).
- In PSCP, you can now use the name of the session instead of
a hostname: type `pscp sessionname:file localfile', where
`sessionname' is replaced by the name of your saved session.
Secondly, you can supply the name of a private key file on the
command line, with the `-i' option. See section 3.7.3.16 for more
information.
Thirdly, PSCP will attempt to authenticate using Pageant if Pageant
is running (see chapter 9). So you would do this:
- Ensure Pageant is running, and has your private key stored in
it.
- Specify a user and host name to PSCP as normal. PSCP will
automatically detect Pageant and try to use the keys within it.
For more general information on public-key authentication, see
chapter 8.
Chapter 6: Using PSFTP to transfer files securely
-------------------------------------------------
PSFTP, the PuTTY SFTP client, is a tool for transferring files
securely between computers using an SSH connection.
PSFTP differs from PSCP in the following ways:
- PSCP should work on virtually every SSH server. PSFTP uses the
new SFTP protocol, which is a feature of SSH-2 only. (PSCP
will also use this protocol if it can, but there is an SSH-1
equivalent it can fall back to if it cannot.)
- PSFTP allows you to run an interactive file transfer session,
much like the Windows `ftp' program. You can list the contents
of directories, browse around the file system, issue multiple
`get' and `put' commands, and eventually log out. By contrast,
PSCP is designed to do a single file transfer operation and
immediately terminate.
6.1 Starting PSFTP
The usual way to start PSFTP is from a command prompt, much like
PSCP. To do this, it will need either to be on your `PATH' or in
your current directory. To add the directory containing PSFTP to
your `PATH' environment variable, type into the console window:
set PATH=C:\path\to\putty\directory;%PATH%
Unlike PSCP, however, PSFTP has no complex command-line syntax; you
just specify a host name and perhaps a user name:
psftp server.example.com
or perhaps
psftp fred@server.example.com
Alternatively, if you just type `psftp' on its own (or double-click
the PSFTP icon in the Windows GUI), you will see the PSFTP prompt,
and a message telling you PSFTP has not connected to any server:
C:\>psftp
psftp: no hostname specified; use "open host.name" to connect
psftp>
At this point you can type `open server.example.com' or
`open fred@server.example.com' to start a session.
PSFTP accepts all the general command line options supported by the
PuTTY tools, except the ones which make no sense in a file transfer
utility. See section 3.7.3 for a description of these options. (The
ones not supported by PSFTP are clearly marked.)
PSFTP also supports some of its own options. The following sections
describe PSFTP's specific command-line options.
6.1.1 `-b': specify a file containing batch commands
In normal operation, PSFTP is an interactive program which displays
a command line and accepts commands from the keyboard.
If you need to do automated tasks with PSFTP, you would probably
prefer to specify a set of commands in advance and have them
executed automatically. The `-b' option allows you to do this. You
use it with a file name containing batch commands. For example, you
might create a file called `myscript.scr' containing lines like
this:
cd /home/ftp/users/jeff
del jam-old.tar.gz
ren jam.tar.gz jam-old.tar.gz
put jam.tar.gz
chmod a+r jam.tar.gz
quit
and then you could run the script by typing
psftp user@hostname -b myscript.scr
When you run a batch script in this way, PSFTP will abort the script
if any command fails to complete successfully. To change this
behaviour, you can add the `-be' option (section 6.1.3).
6.1.2 `-bc': display batch commands as they are run
The `-bc' option alters what PSFTP displays while processing a
batch script specified with `-b'. With the `-bc' option, PSFTP will
display prompts and commands just as if the commands had been typed
at the keyboard. So instead of seeing this:
C:\>psftp fred@hostname -b batchfile
Sent username "fred"
Remote working directory is /home/fred
Listing directory /home/fred/lib
drwxrwsr-x 4 fred fred 1024 Sep 6 10:42 .
drwxr-sr-x 25 fred fred 2048 Dec 14 09:36 ..
drwxrwsr-x 3 fred fred 1024 Apr 17 2000 jed
lrwxrwxrwx 1 fred fred 24 Apr 17 2000 timber
drwxrwsr-x 2 fred fred 1024 Mar 13 2000 trn
you might see this:
C:\>psftp fred@hostname -bc -b batchfile
Sent username "fred"
Remote working directory is /home/fred
psftp> dir lib
Listing directory /home/fred/lib
drwxrwsr-x 4 fred fred 1024 Sep 6 10:42 .
drwxr-sr-x 25 fred fred 2048 Dec 14 09:36 ..
drwxrwsr-x 3 fred fred 1024 Apr 17 2000 jed
lrwxrwxrwx 1 fred fred 24 Apr 17 2000 timber
drwxrwsr-x 2 fred fred 1024 Mar 13 2000 trn
psftp> quit
6.1.3 `-be': continue batch processing on errors
When running a batch file, this additional option causes PSFTP
to continue processing even if a command fails to complete
successfully.
You might want this to happen if you wanted to delete a file and
didn't care if it was already not present, for example.
6.1.4 `-batch': avoid interactive prompts
If you use the `-batch' option, PSFTP will never give an interactive
prompt while establishing the connection. If the server's host key
is invalid, for example (see section 2.2), then the connection will
simply be abandoned instead of asking you what to do next.
This may help PSFTP's behaviour when it is used in automated
scripts: using `-batch', if something goes wrong at connection time,
the batch job will fail rather than hang.
6.2 Running PSFTP
Once you have started your PSFTP session, you will see a `psftp>'
prompt. You can now type commands to perform file-transfer
functions. This section lists all the available commands.
6.2.1 General quoting rules for PSFTP commands
Most PSFTP commands are considered by the PSFTP command interpreter
as a sequence of words, separated by spaces. For example, the
command `ren oldfilename newfilename' splits up into three words:
`ren' (the command name), `oldfilename' (the name of the file to be
renamed), and `newfilename' (the new name to give the file).
Sometimes you will need to specify file names that _contain_ spaces.
In order to do this, you can surround the file name with double
quotes. This works equally well for local file names and remote file
names:
psftp> get "spacey file name.txt" "save it under this name.txt"
The double quotes themselves will not appear as part of the file
names; they are removed by PSFTP and their only effect is to stop
the spaces inside them from acting as word separators.
If you need to _use_ a double quote (on some types of remote system,
such as Unix, you are allowed to use double quotes in file names),
you can do this by doubling it. This works both inside and outside
double quotes. For example, this command
psftp> ren ""this"" "a file with ""quotes"" in it"
will take a file whose current name is `"this"' (with a double quote
character at the beginning and the end) and rename it to a file
whose name is `a file with "quotes" in it'.
(The one exception to the PSFTP quoting rules is the `!' command,
which passes its command line straight to Windows without splitting
it up into words at all. See section 6.2.19.)
6.2.2 Wildcards in PSFTP
Several commands in PSFTP support `wildcards' to select multiple
files.
For _local_ file specifications (such as the first argument to
`put'), wildcard rules for the local operating system are used. For
instance, PSFTP running on Windows might require the use of `*.*'
where PSFTP on Unix would need `*'.
For _remote_ file specifications (such as the first argument to
`get'), PSFTP uses a standard wildcard syntax (similar to POSIX
wildcards):
- `*' matches any sequence of characters (including a zero-length
sequence).
- `?' matches exactly one character.
- `[abc]' matches exactly one character which can be a, b, or c.
`[a-z]' matches any character in the range a to z.
`[^abc]' matches a single character that is _not_ a, b, or c.
Special cases: `[-a]' matches a literal hyphen (-) or a; `[^-a]'
matches all other characters. `[a^]' matches a literal caret (^)
or a.
- `\' (backslash) before any of the above characters (or itself)
removes that character's special meaning.
A leading period (.) on a filename is not treated specially, unlike
in some Unix contexts; `get *' will fetch all files, whether or not
they start with a leading period.
6.2.3 The `open' command: start a session
If you started PSFTP by double-clicking in the GUI, or just by
typing `psftp' at the command line, you will need to open a
connection to an SFTP server before you can issue any other commands
(except `help' and `quit').
To create a connection, type `open host.name', or if you need to
specify a user name as well you can type `open user@host.name'.
Once you have issued this command, you will not be able to issue it
again, _even_ if the command fails (for example, if you mistype the
host name or the connection times out). So if the connection is not
opened successfully, PSFTP will terminate immediately.
6.2.4 The `quit' command: end your session
When you have finished your session, type the command `quit' to
close the connection, terminate PSFTP and return to the command line
(or just close the PSFTP console window if you started it from the
GUI).
You can also use the `bye' and `exit' commands, which have exactly
the same effect.
6.2.5 The `close' command: close your connection
If you just want to close the network connection but keep PSFTP
running, you can use the `close' command. You can then use the
`open' command to open a new connection.
6.2.6 The `help' command: get quick online help
If you type `help', PSFTP will give a short list of the available
commands.
If you type `help' with a command name - for example, `help get'
- then PSFTP will give a short piece of help on that particular
command.
6.2.7 The `cd' and `pwd' commands: changing the remote working directory
PSFTP maintains a notion of your `working directory' on the server.
This is the default directory that other commands will operate on.
For example, if you type `get filename.dat' then PSFTP will look for
`filename.dat' in your remote working directory on the server.
To change your remote working directory, use the `cd' command. If
you don't provide an argument, `cd' will return you to your home
directory on the server (more precisely, the remote directory you
were in at the start of the connection).
To display your current remote working directory, type `pwd'.
6.2.8 The `lcd' and `lpwd' commands: changing the local working directory
As well as having a working directory on the remote server, PSFTP
also has a working directory on your local machine (just like
any other Windows process). This is the default local directory
that other commands will operate on. For example, if you type
`get filename.dat' then PSFTP will save the resulting file as
`filename.dat' in your local working directory.
To change your local working directory, use the `lcd' command. To
display your current local working directory, type `lpwd'.
6.2.9 The `get' command: fetch a file from the server
To download a file from the server and store it on your local PC,
you use the `get' command.
In its simplest form, you just use this with a file name:
get myfile.dat
If you want to store the file locally under a different name,
specify the local file name after the remote one:
get myfile.dat newname.dat
This will fetch the file on the server called `myfile.dat', but will
save it to your local machine under the name `newname.dat'.
To fetch an entire directory recursively, you can use the `-r'
option:
get -r mydir
get -r mydir newname
(If you want to fetch a file whose name starts with a hyphen, you
may have to use the `--' special argument, which stops `get' from
interpreting anything as a switch after it. For example, `get -- -
silly-name-'.)
6.2.10 The `put' command: send a file to the server
To upload a file to the server from your local PC, you use the `put'
command.
In its simplest form, you just use this with a file name:
put myfile.dat
If you want to store the file remotely under a different name,
specify the remote file name after the local one:
put myfile.dat newname.dat
This will send the local file called `myfile.dat', but will store it
on the server under the name `newname.dat'.
To send an entire directory recursively, you can use the `-r'
option:
put -r mydir
put -r mydir newname
(If you want to send a file whose name starts with a hyphen, you
may have to use the `--' special argument, which stops `put' from
interpreting anything as a switch after it. For example, `put -- -
silly-name-'.)
6.2.11 The `mget' and `mput' commands: fetch or send multiple files
`mget' works almost exactly like `get', except that it allows you to
specify more than one file to fetch at once. You can do this in two
ways:
- by giving two or more explicit file names
(`mget file1.txt file2.txt')
- by using a wildcard (`mget *.txt').
Every argument to `mget' is treated as the name of a file to fetch
(unlike `get', which will interpret at most one argument like
that, and a second argument will be treated as an alternative name
under which to store the retrieved file), or a wildcard expression
matching more than one file.
The `-r' and `--' options from `get' are also available with `mget'.
`mput' is similar to `put', with the same differences.
6.2.12 The `reget' and `reput' commands: resuming file transfers
If a file transfer fails half way through, and you end up with half
the file stored on your disk, you can resume the file transfer using
the `reget' and `reput' commands. These work exactly like the `get'
and `put' commands, but they check for the presence of the half-
written destination file and start transferring from where the last
attempt left off.
The syntax of `reget' and `reput' is exactly the same as the syntax
of `get' and `put':
reget myfile.dat
reget myfile.dat newname.dat
reget -r mydir
These commands are intended mainly for resuming interrupted
transfers. They assume that the remote file or directory structure
has not changed in any way; if there have been changes, you may end
up with corrupted files. In particular, the `-r' option will not
pick up changes to files or directories already transferred in full.
6.2.13 The `dir' command: list remote files
To list the files in your remote working directory, just type `dir'.
You can also list the contents of a different directory by typing
`dir' followed by the directory name:
dir /home/fred
dir sources
And you can list a subset of the contents of a directory by
providing a wildcard:
dir /home/fred/*.txt
dir sources/*.c
The `ls' command works exactly the same way as `dir'.
6.2.14 The `chmod' command: change permissions on remote files
PSFTP allows you to modify the file permissions on files and
directories on the server. You do this using the `chmod' command,
which works very much like the Unix `chmod' command.
The basic syntax is `chmod modes file', where `modes' represents a
modification to the file permissions, and `file' is the filename to
modify. You can specify multiple files or wildcards. For example:
chmod go-rwx,u+w privatefile
chmod a+r public*
chmod 640 groupfile1 groupfile2
The `modes' parameter can be a set of octal digits in the Unix
style. (If you don't know what this means, you probably don't want
to be using it!) Alternatively, it can be a list of permission
modifications, separated by commas. Each modification consists of:
- The people affected by the modification. This can be `u'
(the owning user), `g' (members of the owning group), or `o'
(everybody else - `others'), or some combination of those. It
can also be `a' (`all') to affect everybody at once.
- A `+' or `-' sign, indicating whether permissions are to be
added or removed.
- The actual permissions being added or removed. These can be
`r' (permission to read the file), `w' (permission to write
to the file), and `x' (permission to execute the file, or in
the case of a directory, permission to access files within the
directory).
So the above examples would do:
- The first example: `go-rwx' removes read, write and execute
permissions for members of the owning group and everybody else
(so the only permissions left are the ones for the file owner).
`u+w' adds write permission for the file owner.
- The second example: `a+r' adds read permission for everybody to
all files and directories starting with `public'.
In addition to all this, there are a few extra special cases for
Unix systems. On non-Unix systems these are unlikely to be useful:
- You can specify `u+s' and `u-s' to add or remove the Unix set-
user-ID bit. This is typically only useful for special purposes;
refer to your Unix documentation if you're not sure about it.
- You can specify `g+s' and `g-s' to add or remove the Unix set-
group-ID bit. On a file, this works similarly to the set-user-
ID bit (see your Unix documentation again); on a directory it
ensures that files created in the directory are accessible by
members of the group that owns the directory.
- You can specify `+t' and `-t' to add or remove the Unix `sticky
bit'. When applied to a directory, this means that the owner of
a file in that directory can delete the file (whereas normally
only the owner of the _directory_ would be allowed to).
6.2.15 The `del' command: delete remote files
To delete a file on the server, type `del' and then the filename or
filenames:
del oldfile.dat
del file1.txt file2.txt
del *.o
Files will be deleted without further prompting, even if multiple
files are specified.
`del' will only delete files. You cannot use it to delete
directories; use `rmdir' for that.
The `rm' command works exactly the same way as `del'.
6.2.16 The `mkdir' command: create remote directories
To create a directory on the server, type `mkdir' and then the
directory name:
mkdir newstuff
You can specify multiple directories to create at once:
mkdir dir1 dir2 dir3
6.2.17 The `rmdir' command: remove remote directories
To remove a directory on the server, type `rmdir' and then the
directory name or names:
rmdir oldstuff
rmdir *.old ancient
Directories will be deleted without further prompting, even if
multiple directories are specified.
Most SFTP servers will probably refuse to remove a directory if
the directory has anything in it, so you will need to delete the
contents first.
6.2.18 The `mv' command: move and rename remote files
To rename a single file on the server, type `mv', then the current
file name, and then the new file name:
mv oldfile newname
You can also move the file into a different directory and change the
name:
mv oldfile dir/newname
To move one or more files into an existing subdirectory, specify
the files (using wildcards if desired), and then the destination
directory:
mv file dir
mv file1 dir1/file2 dir2
mv *.c *.h ..
The `rename' and `ren' commands work exactly the same way as `mv'.
6.2.19 The `!' command: run a local Windows command
You can run local Windows commands using the `!' command. This is
the only PSFTP command that is not subject to the command quoting
rules given in section 6.2.1. If any command line begins with the
`!' character, then the rest of the line will be passed straight to
Windows without further translation.
For example, if you want to move an existing copy of a file out of
the way before downloading an updated version, you might type:
psftp> !ren myfile.dat myfile.bak
psftp> get myfile.dat
using the Windows `ren' command to rename files on your local PC.
6.3 Using public key authentication with PSFTP
Like PuTTY, PSFTP can authenticate using a public key instead of a
password. There are three ways you can do this.
Firstly, PSFTP can use PuTTY saved sessions in place of hostnames.
So you might do this:
- Run PuTTY, and create a PuTTY saved session (see section 4.1.2)
which specifies your private key file (see section 4.20.5). You
will probably also want to specify a username to log in as (see
section 4.14.1).
- In PSFTP, you can now use the name of the session instead of
a hostname: type `psftp sessionname', where `sessionname' is
replaced by the name of your saved session.
Secondly, you can supply the name of a private key file on the
command line, with the `-i' option. See section 3.7.3.16 for more
information.
Thirdly, PSFTP will attempt to authenticate using Pageant if Pageant
is running (see chapter 9). So you would do this:
- Ensure Pageant is running, and has your private key stored in
it.
- Specify a user and host name to PSFTP as normal. PSFTP will
automatically detect Pageant and try to use the keys within it.
For more general information on public-key authentication, see
chapter 8.
Chapter 7: Using the command-line connection tool Plink
-------------------------------------------------------
Plink (PuTTY Link) is a command-line connection tool similar to UNIX
`ssh'. It is mostly used for automated operations, such as making
CVS access a repository on a remote server.
Plink is probably not what you want if you want to run an
interactive session in a console window.
7.1 Starting Plink
Plink is a command line application. This means that you cannot just
double-click on its icon to run it and instead you have to bring
up a console window. In Windows 95, 98, and ME, this is called an
`MS-DOS Prompt', and in Windows NT, 2000, and XP, it is called a
`Command Prompt'. It should be available from the Programs section
of your Start Menu.
In order to use Plink, the file `plink.exe' will need either to be
on your `PATH' or in your current directory. To add the directory
containing Plink to your `PATH' environment variable, type into the
console window:
set PATH=C:\path\to\putty\directory;%PATH%
This will only work for the lifetime of that particular console
window. To set your `PATH' more permanently on Windows NT, 2000, and
XP, use the Environment tab of the System Control Panel. On Windows
95, 98, and ME, you will need to edit your `AUTOEXEC.BAT' to include
a `set' command like the one above.
7.2 Using Plink
This section describes the basics of how to use Plink for
interactive logins and for automated processes.
Once you've got a console window to type into, you can just type
`plink' on its own to bring up a usage message. This tells you the
version of Plink you're using, and gives you a brief summary of how
to use Plink:
Z:\sysosd>plink
PuTTY Link: command-line connection utility
Release 0.58
Usage: plink [options] [user@]host [command]
("host" can also be a PuTTY saved session name)
Options:
-V print version information and exit
-pgpfp print PGP key fingerprints and exit
-v show verbose messages
-load sessname Load settings from saved session
-ssh -telnet -rlogin -raw
force use of a particular protocol
-P port connect to specified port
-l user connect with specified username
-batch disable all interactive prompts
The following options only apply to SSH connections:
-pw passw login with specified password
-D [listen-IP:]listen-port
Dynamic SOCKS-based port forwarding
-L [listen-IP:]listen-port:host:port
Forward local port to remote address
-R [listen-IP:]listen-port:host:port
Forward remote port to local address
-X -x enable / disable X11 forwarding
-A -a enable / disable agent forwarding
-t -T enable / disable pty allocation
-1 -2 force use of particular protocol version
-4 -6 force use of IPv4 or IPv6
-C enable compression
-i key private key file for authentication
-m file read remote command(s) from file
-s remote command is an SSH subsystem (SSH-2 only)
-N don't start a shell/command (SSH-2 only)
Once this works, you are ready to use Plink.
7.2.1 Using Plink for interactive logins
To make a simple interactive connection to a remote server, just
type `plink' and then the host name:
Z:\sysosd>plink login.example.com
Debian GNU/Linux 2.2 flunky.example.com
flunky login:
You should then be able to log in as normal and run a session. The
output sent by the server will be written straight to your command
prompt window, which will most likely not interpret terminal control
codes in the way the server expects it to. So if you run any full-
screen applications, for example, you can expect to see strange
characters appearing in your window. Interactive connections like
this are not the main point of Plink.
In order to connect with a different protocol, you can give the
command line options `-ssh', `-telnet', `-rlogin' or `-raw'. To make
an SSH connection, for example:
Z:\sysosd>plink -ssh login.example.com
login as:
If you have already set up a PuTTY saved session, then instead of
supplying a host name, you can give the saved session name. This
allows you to use public-key authentication, specify a user name,
and use most of the other features of PuTTY:
Z:\sysosd>plink my-ssh-session
Sent username "fred"
Authenticating with public key "fred@winbox"
Last login: Thu Dec 6 19:25:33 2001 from :0.0
fred@flunky:~$
(You can also use the `-load' command-line option to load a saved
session; see section 3.7.3.1. If you use `-load', the saved session
exists, and it specifies a hostname, you cannot also specify a
`host' or `user@host' argument - it will be treated as part of the
remote command.)
7.2.2 Using Plink for automated connections
More typically Plink is used with the SSH protocol, to enable you
to talk directly to a program running on the server. To do this you
have to ensure Plink is _using_ the SSH protocol. You can do this in
several ways:
- Use the `-ssh' option as described in section 7.2.1.
- Set up a PuTTY saved session that describes the server you are
connecting to, and that also specifies the protocol as SSH.
- Set the Windows environment variable `PLINK_PROTOCOL' to the
word `ssh'.
Usually Plink is not invoked directly by a user, but run
automatically by another process. Therefore you typically do not
want Plink to prompt you for a user name or a password.
Next, you are likely to need to avoid the various interactive
prompts Plink can produce. You might be prompted to verify the host
key of the server you're connecting to, to enter a user name, or to
enter a password.
To avoid being prompted for the server host key when using Plink for
an automated connection, you should first make a _manual_ connection
(using either of PuTTY or Plink) to the same server, verify the host
key (see section 2.2 for more information), and select Yes to add
the host key to the Registry. After that, Plink commands connecting
to that server should not give a host key prompt unless the host key
changes.
To avoid being prompted for a user name, you can:
- Use the `-l' option to specify a user name on the command line.
For example, `plink login.example.com -l fred'.
- Set up a PuTTY saved session that describes the server you are
connecting to, and that also specifies the username to log in as
(see section 4.14.1).
To avoid being prompted for a password, you should almost certainly
set up public-key authentication. (See chapter 8 for a general
introduction to public-key authentication.) Again, you can do this
in two ways:
- Set up a PuTTY saved session that describes the server you
are connecting to, and that also specifies a private key file
(see section 4.20.5). For this to work without prompting, your
private key will need to have no passphrase.
- Store the private key in Pageant. See chapter 9 for further
information.
Once you have done all this, you should be able to run a remote
command on the SSH server machine and have it execute automatically
with no prompting:
Z:\sysosd>plink login.example.com -l fred echo hello, world
hello, world
Z:\sysosd>
Or, if you have set up a saved session with all the connection
details:
Z:\sysosd>plink mysession echo hello, world
hello, world
Z:\sysosd>
Then you can set up other programs to run this Plink command and
talk to it as if it were a process on the server machine.
7.2.3 Plink command line options
Plink accepts all the general command line options supported by the
PuTTY tools. See section 3.7.3 for a description of these options.
Plink also supports some of its own options. The following sections
describe Plink's specific command-line options.
7.2.3.1 `-batch': disable all interactive prompts
If you use the `-batch' option, Plink will never give an interactive
prompt while establishing the connection. If the server's host key
is invalid, for example (see section 2.2), then the connection will
simply be abandoned instead of asking you what to do next.
This may help Plink's behaviour when it is used in automated
scripts: using `-batch', if something goes wrong at connection time,
the batch job will fail rather than hang.
7.2.3.2 `-s': remote command is SSH subsystem
If you specify the `-s' option, Plink passes the specified command
as the name of an SSH `subsystem' rather than an ordinary command
line.
(This option is only meaningful with the SSH-2 protocol.)
7.3 Using Plink in batch files and scripts
Once you have set up Plink to be able to log in to a remote server
without any interactive prompting (see section 7.2.2), you can use
it for lots of scripting and batch purposes. For example, to start a
backup on a remote machine, you might use a command like:
plink root@myserver /etc/backups/do-backup.sh
Or perhaps you want to fetch all system log lines relating to a
particular web area:
plink mysession grep /~fred/ /var/log/httpd/access.log > fredlog
Any non-interactive command you could usefully run on the server
command line, you can run in a batch file using Plink in this way.
7.4 Using Plink with CVS
To use Plink with CVS, you need to set the environment variable
`CVS_RSH' to point to Plink:
set CVS_RSH=\path\to\plink.exe
You also need to arrange to be able to connect to a remote host
without any interactive prompts, as described in section 7.2.2.
You should then be able to run CVS as follows:
cvs -d :ext:user@sessionname:/path/to/repository co module
If you specified a username in your saved session, you don't even
need to specify the `user' part of this, and you can just say:
cvs -d :ext:sessionname:/path/to/repository co module
7.5 Using Plink with WinCVS
Plink can also be used with WinCVS. Firstly, arrange for Plink to be
able to connect to a remote host non-interactively, as described in
section 7.2.2.
Then, in WinCVS, bring up the `Preferences' dialogue box from the
_Admin_ menu, and switch to the `Ports' tab. Tick the box there
labelled `Check for an alternate rsh name' and in the text entry
field to the right enter the full path to `plink.exe'. Select `OK'
on the `Preferences' dialogue box.
Next, select `Command Line' from the WinCVS `Admin' menu, and type a
CVS command as in section 7.4, for example:
cvs -d :ext:user@hostname:/path/to/repository co module
or (if you're using a saved session):
cvs -d :ext:user@sessionname:/path/to/repository co module
Select the folder you want to check out to with the `Change Folder'
button, and click `OK' to check out your module. Once you've got
modules checked out, WinCVS will happily invoke plink from the GUI
for CVS operations.
Chapter 8: Using public keys for SSH authentication
---------------------------------------------------
8.1 Public key authentication - an introduction
Public key authentication is an alternative means of identifying
yourself to a login server, instead of typing a password. It is more
secure and more flexible, but more difficult to set up.
In conventional password authentication, you prove you are who you
claim to be by proving that you know the correct password. The
only way to prove you know the password is to tell the server what
you think the password is. This means that if the server has been
hacked, or _spoofed_ (see section 2.2), an attacker can learn your
password.
Public key authentication solves this problem. You generate a _key
pair_, consisting of a public key (which everybody is allowed to
know) and a private key (which you keep secret and do not give
to anybody). The private key is able to generate _signatures_. A
signature created using your private key cannot be forged by anybody
who does not have that key; but anybody who has your public key can
verify that a particular signature is genuine.
So you generate a key pair on your own computer, and you copy the
public key to the server. Then, when the server asks you to prove
who you are, PuTTY can generate a signature using your private key.
The server can verify that signature (since it has your public key)
and allow you to log in. Now if the server is hacked or spoofed, the
attacker does not gain your private key or password; they only gain
one signature. And signatures cannot be re-used, so they have gained
nothing.
There is a problem with this: if your private key is stored
unprotected on your own computer, then anybody who gains access to
_that_ will be able to generate signatures as if they were you. So
they will be able to log in to your server under your account. For
this reason, your private key is usually _encrypted_ when it is
stored on your local machine, using a passphrase of your choice. In
order to generate a signature, PuTTY must decrypt the key, so you
have to type your passphrase.
This can make public-key authentication less convenient than
password authentication: every time you log in to the server,
instead of typing a short password, you have to type a longer
passphrase. One solution to this is to use an _authentication
agent_, a separate program which holds decrypted private keys and
generates signatures on request. PuTTY's authentication agent is
called Pageant. When you begin a Windows session, you start Pageant
and load your private key into it (typing your passphrase once).
For the rest of your session, you can start PuTTY any number of
times and Pageant will automatically generate signatures without you
having to do anything. When you close your Windows session, Pageant
shuts down, without ever having stored your decrypted private key on
disk. Many people feel this is a good compromise between security
and convenience. See chapter 9 for further details.
There is more than one public-key algorithm available. The most
common is RSA, but others exist, notably DSA (otherwise known as
DSS), the USA's federal Digital Signature Standard. The key types
supported by PuTTY are described in section 8.2.2.
8.2 Using PuTTYgen, the PuTTY key generator
PuTTYgen is a key generator. It generates pairs of public and
private keys to be used with PuTTY, PSCP, and Plink, as well as
the PuTTY authentication agent, Pageant (see chapter 9). PuTTYgen
generates RSA and DSA keys.
When you run PuTTYgen you will see a window where you have two
choices: `Generate', to generate a new public/private key pair, or
`Load' to load in an existing private key.
8.2.1 Generating a new key
This is a general outline of the procedure for generating a new key
pair. The following sections describe the process in more detail.
- First, you need to select which type of key you want to
generate, and also select the strength of the key. This is
described in more detail in section 8.2.2 and section 8.2.3.
- Then press the `Generate' button, to actually generate the key.
Section 8.2.4 describes this step.
- Once you have generated the key, select a comment field (section
8.2.6) and a passphrase (section 8.2.7).
- Now you're ready to save the private key to disk; press the
`Save private key' button. (See section 8.2.8).
Your key pair is now ready for use. You may also want to copy the
public key to your server, either by copying it out of the `Public
key for pasting into authorized_keys file' box (see section 8.2.10),
or by using the `Save public key' button (section 8.2.9). However,
you don't need to do this immediately; if you want, you can load the
private key back into PuTTYgen later (see section 8.2.11) and the
public key will be available for copying and pasting again.
Section 8.3 describes the typical process of configuring PuTTY to
attempt public-key authentication, and configuring your SSH server
to accept it.
8.2.2 Selecting the type of key
Before generating a key pair using PuTTYgen, you need to select
which type of key you need. PuTTYgen currently supports three types
of key:
- An RSA key for use with the SSH-1 protocol.
- An RSA key for use with the SSH-2 protocol.
- A DSA key for use with the SSH-2 protocol.
The SSH-1 protocol only supports RSA keys; if you will be connecting
using the SSH-1 protocol, you must select the first key type or your
key will be completely useless.
The SSH-2 protocol supports more than one key type. The two types
supported by PuTTY are RSA and DSA.
The PuTTY developers _strongly_ recommend you use RSA. DSA has an
intrinsic weakness which makes it very easy to create a signature
which contains enough information to give away the _private_ key!
This would allow an attacker to pretend to be you for any number
of future sessions. PuTTY's implementation has taken very careful
precautions to avoid this weakness, but we cannot be 100% certain we
have managed it, and if you have the choice we strongly recommend
using RSA keys instead.
If you really need to connect to an SSH server which only supports
DSA, then you probably have no choice but to use DSA. If you do use
DSA, we recommend you do not use the same key to authenticate with
more than one server.
8.2.3 Selecting the size (strength) of the key
The `Number of bits' input box allows you to choose the strength of
the key PuTTYgen will generate.
Currently 1024 bits should be sufficient for most purposes.
Note that an RSA key is generated by finding two primes of half the
length requested, and then multiplying them together. For example,
if you ask PuTTYgen for a 1024-bit RSA key, it will create two 512-
bit primes and multiply them. The result of this multiplication
might be 1024 bits long, or it might be only 1023; so you may not
get the exact length of key you asked for. This is perfectly normal,
and you do not need to worry. The lengths should only ever differ by
one, and there is no perceptible drop in security as a result.
DSA keys are not created by multiplying primes together, so they
should always be exactly the length you asked for.
8.2.4 The `Generate' button
Once you have chosen the type of key you want, and the strength of
the key, press the `Generate' button and PuTTYgen will begin the
process of actually generating the key.
First, a progress bar will appear and PuTTYgen will ask you to move
the mouse around to generate randomness. Wave the mouse in circles
over the blank area in the PuTTYgen window, and the progress bar
will gradually fill up as PuTTYgen collects enough randomness. You
don't need to wave the mouse in particularly imaginative patterns
(although it can't hurt); PuTTYgen will collect enough randomness
just from the fine detail of _exactly_ how far the mouse has moved
each time Windows samples its position.
When the progress bar reaches the end, PuTTYgen will begin creating
the key. The progress bar will reset to the start, and gradually
move up again to track the progress of the key generation. It will
not move evenly, and may occasionally slow down to a stop; this
is unfortunately unavoidable, because key generation is a random
process and it is impossible to reliably predict how long it will
take.
When the key generation is complete, a new set of controls will
appear in the window to indicate this.
8.2.5 The `Key fingerprint' box
The `Key fingerprint' box shows you a fingerprint value for the
generated key. This is derived cryptographically from the _public_
key value, so it doesn't need to be kept secret.
The fingerprint value is intended to be cryptographically secure,
in the sense that it is computationally infeasible for someone to
invent a second key with the same fingerprint, or to find a key with
a particular fingerprint. So some utilities, such as the Pageant key
list box (see section 9.2.1) and the Unix `ssh-add' utility, will
list key fingerprints rather than the whole public key.
8.2.6 Setting a comment for your key
If you have more than one key and use them for different purposes,
you don't need to memorise the key fingerprints in order to tell
them apart. PuTTY allows you to enter a _comment_ for your key,
which will be displayed whenever PuTTY or Pageant asks you for the
passphrase.
The default comment format, if you don't specify one, contains the
key type and the date of generation, such as `rsa-key-20011212'.
Another commonly used approach is to use your name and the name of
the computer the key will be used on, such as `simon@simons-pc'.
To alter the key comment, just type your comment text into the `Key
comment' box before saving the private key. If you want to change
the comment later, you can load the private key back into PuTTYgen,
change the comment, and save it again.
8.2.7 Setting a passphrase for your key
The `Key passphrase' and `Confirm passphrase' boxes allow you to
choose a passphrase for your key. The passphrase will be used to
encrypt the key on disk, so you will not be able to use the key
without first entering the passphrase.
When you save the key, PuTTY will check that the `Key passphrase'
and `Confirm passphrase' boxes both contain exactly the same
passphrase, and will refuse to save the key otherwise.
If you leave the passphrase fields blank, the key will be saved
unencrypted. You should _not_ do this without good reason; if you
do, your private key file on disk will be all an attacker needs to
gain access to any machine configured to accept that key. If you
want to be able to passwordless loginlog in without having to type
a passphrase every time, you should consider using Pageant (chapter
9) so that your decrypted key is only held in memory rather than on
disk.
Under special circumstances you may genuinely _need_ to use a key
with no passphrase; for example, if you need to run an automated
batch script that needs to make an SSH connection, you can't be
there to type the passphrase. In this case we recommend you generate
a special key for each specific batch script (or whatever) that
needs one, and on the server side you should arrange that each
key is _restricted_ so that it can only be used for that specific
purpose. The documentation for your SSH server should explain how to
do this (it will probably vary between servers).
Choosing a good passphrase is difficult. Just as you shouldn't use
a dictionary word as a password because it's easy for an attacker
to run through a whole dictionary, you should not use a song
lyric, quotation or other well-known sentence as a passphrase.
DiceWare (www.diceware.com) recommends using at least five words
each generated randomly by rolling five dice, which gives over
2^64 possible passphrases and is probably not a bad scheme. If you
want your passphrase to make grammatical sense, this cuts down the
possibilities a lot and you should use a longer one as a result.
_Do not forget your passphrase_. There is no way to recover it.
8.2.8 Saving your private key to a disk file
Once you have generated a key, set a comment field and set a
passphrase, you are ready to save your private key to disk.
Press the `Save private key' button. PuTTYgen will put up a dialog
box asking you where to save the file. Select a directory, type in a
file name, and press `Save'.
This file is in PuTTY's native format (`*.'`PPK'); it is the one
you will need to tell PuTTY to use for authentication (see section
4.20.5) or tell Pageant to load (see section 9.2.2).
8.2.9 Saving your public key to a disk file
The SSH-2 protocol drafts specify a standard format for storing
public keys on disk. Some SSH servers (such as ssh.com's) require a
public key in this format in order to accept authentication with the
corresponding private key. (Others, such as OpenSSH, use a different
format; see section 8.2.10.)
To save your public key in the SSH-2 standard format, press the
`Save public key' button in PuTTYgen. PuTTYgen will put up a dialog
box asking you where to save the file. Select a directory, type in a
file name, and press `Save'.
You will then probably want to copy the public key file to your
SSH server machine. See section 8.3 for general instructions on
configuring public-key authentication once you have generated a key.
If you use this option with an SSH-1 key, the file PuTTYgen saves
will contain exactly the same text that appears in the `Public key
for pasting' box. This is the only existing standard for SSH-1
public keys.
8.2.10 `Public key for pasting into authorized_keys file'
All SSH-1 servers require your public key to be given to it in a
one-line format before it will accept authentication with your
private key. The OpenSSH server also requires this for SSH-2.
The `Public key for pasting into authorized_keys file' gives the
public-key data in the correct one-line format. Typically you will
want to select the entire contents of the box using the mouse, press
Ctrl+C to copy it to the clipboard, and then paste the data into a
PuTTY session which is already connected to the server.
See section 8.3 for general instructions on configuring public-key
authentication once you have generated a key.
8.2.11 Reloading a private key
PuTTYgen allows you to load an existing private key file into
memory. If you do this, you can then change the passphrase and
comment before saving it again; you can also make extra copies of
the public key.
To load an existing key, press the `Load' button. PuTTYgen will put
up a dialog box where you can browse around the file system and find
your key file. Once you select the file, PuTTYgen will ask you for a
passphrase (if necessary) and will then display the key details in
the same way as if it had just generated the key.
If you use the Load command to load a foreign key format, it will
work, but you will see a message box warning you that the key you
have loaded is not a PuTTY native key. See section 8.2.12 for
information about importing foreign key formats.
8.2.12 Dealing with private keys in other formats
Most SSH-1 clients use a standard format for storing private keys on
disk. PuTTY uses this format as well; so if you have generated an
SSH-1 private key using OpenSSH or ssh.com's client, you can use it
with PuTTY, and vice versa.
However, SSH-2 private keys have no standard format. OpenSSH and
ssh.com have different formats, and PuTTY's is different again. So
a key generated with one client cannot immediately be used with
another.
Using the `Import' command from the `Conversions' menu, PuTTYgen can
load SSH-2 private keys in OpenSSH's format and ssh.com's format.
Once you have loaded one of these key types, you can then save it
back out as a PuTTY-format key (`*.'`PPK') so that you can use it
with the PuTTY suite. The passphrase will be unchanged by this
process (unless you deliberately change it). You may want to change
the key comment before you save the key, since OpenSSH's SSH-2 key
format contains no space for a comment and ssh.com's default comment
format is long and verbose.
PuTTYgen can also export private keys in OpenSSH format and in
ssh.com format. To do so, select one of the `Export' options from
the `Conversions' menu. Exporting a key works exactly like saving
it (see section 8.2.8) - you need to have typed your passphrase in
beforehand, and you will be warned if you are about to save a key
without a passphrase.
Note that since only SSH-2 keys come in different formats, the
export options are not available if you have generated an SSH-1 key.
8.3 Getting ready for public key authentication
Connect to your SSH server using PuTTY with the SSH protocol. When
the connection succeeds you will be prompted for your user name and
password to login. Once logged in, you must configure the server to
accept your public key for authentication:
- If your server is using the SSH-1 protocol, you should change
into the `.ssh' directory and open the file `authorized_keys'
with your favourite editor. (You may have to create this file if
this is the first key you have put in it). Then switch to the
PuTTYgen window, select all of the text in the `Public key for
pasting into authorized_keys file' box (see section 8.2.10), and
copy it to the clipboard (`Ctrl+C'). Then, switch back to the
PuTTY window and insert the data into the open file, making sure
it ends up all on one line. Save the file.
- If your server is OpenSSH and is using the SSH-2 protocol,
you should follow the same instructions, except that
in earlier versions of OpenSSH 2 the file might be
called `authorized_keys2'. (In modern versions the same
`authorized_keys' file is used for both SSH-1 and SSH-2 keys.)
- If your server is ssh.com's product and is using SSH-2, you need
to save a _public_ key file from PuTTYgen (see section 8.2.9),
and copy that into the `.ssh2' directory on the server. Then you
should go into that `.ssh2' directory, and edit (or create) a
file called `authorization'. In this file you should put a line
like `Key mykey.pub', with `mykey.pub' replaced by the name of
your key file.
- For other SSH server software, you should refer to the manual
for that server.
You may also need to ensure that your home directory, your `.ssh'
directory, and any other files involved (such as `authorized_keys',
`authorized_keys2' or `authorization') are not group-writable or
world-writable. You can typically do this by using a command such as
chmod go-w $HOME $HOME/.ssh $HOME/.ssh/authorized_keys
Your server should now be configured to accept authentication using
your private key. Now you need to configure PuTTY to _attempt_
authentication using your private key. You can do this in any of
three ways:
- Select the private key in PuTTY's configuration. See section
4.20.5 for details.
- Specify the key file on the command line with the `-i' option.
See section 3.7.3.16 for details.
- Load the private key into Pageant (see chapter 9). In this case
PuTTY will automatically try to use it for authentication if it
can.
Chapter 9: Using Pageant for authentication
-------------------------------------------
Pageant is an SSH authentication agent. It holds your private keys
in memory, already decoded, so that you can use them often without
needing to type a passphrase.
9.1 Getting started with Pageant
Before you run Pageant, you need to have a private key in `*.'`PPK'
format. See chapter 8 to find out how to generate and use one.
When you run Pageant, it will put an icon of a computer wearing a
hat into the System tray. It will then sit and do nothing, until you
load a private key into it.
If you click the Pageant icon with the right mouse button, you will
see a menu. Select `View Keys' from this menu. The Pageant main
window will appear. (You can also bring this window up by double-
clicking on the Pageant icon.)
The Pageant window contains a list box. This shows the private keys
Pageant is holding. When you start Pageant, it has no keys, so the
list box will be empty. After you add one or more keys, they will
show up in the list box.
To add a key to Pageant, press the `Add Key' button. Pageant will
bring up a file dialog, labelled `Select Private Key File'. Find
your private key file in this dialog, and press `Open'.
Pageant will now load the private key. If the key is protected by
a passphrase, Pageant will ask you to type the passphrase. When
the key has been loaded, it will appear in the list in the Pageant
window.
Now start PuTTY and open an SSH session to a site that accepts your
key. PuTTY will notice that Pageant is running, retrieve the key
automatically from Pageant, and use it to authenticate. You can now
open as many PuTTY sessions as you like without having to type your
passphrase again.
When you want to shut down Pageant, click the right button on the
Pageant icon in the System tray, and select `Exit' from the menu.
Closing the Pageant main window does _not_ shut down Pageant.
9.2 The Pageant main window
The Pageant main window appears when you left-click on the Pageant
system tray icon, or alternatively right-click and select `View
Keys' from the menu. You can use it to keep track of what keys are
currently loaded into Pageant, and to add new ones or remove the
existing keys.
9.2.1 The key list box
The large list box in the Pageant main window lists the private
keys that are currently loaded into Pageant. The list might look
something like this:
ssh1 1024 22:c3:68:3b:09:41:36:c3:39:83:91:ae:71:b2:0f:04 k1
ssh-rsa 1023 74:63:08:82:95:75:e1:7c:33:31:bb:cb:00:c0:89:8b k2
For each key, the list box will tell you:
- The type of the key. Currently, this can be `ssh1' (an RSA key
for use with the SSH-1 protocol), `ssh-rsa' (an RSA key for use
with the SSH-2 protocol), or `ssh-dss' (a DSA key for use with
the SSH-2 protocol).
- The size (in bits) of the key.
- The fingerprint for the public key. This should be the same
fingerprint given by PuTTYgen, and (hopefully) also the same
fingerprint shown by remote utilities such as `ssh-keygen' when
applied to your `authorized_keys' file.
- The comment attached to the key.
9.2.2 The `Add Key' button
To add a key to Pageant by reading it out of a local disk file,
press the `Add Key' button in the Pageant main window, or
alternatively right-click on the Pageant icon in the system tray and
select `Add Key' from there.
Pageant will bring up a file dialog, labelled `Select Private Key
File'. Find your private key file in this dialog, and press `Open'.
If you want to add more than one key at once, you can select
multiple files using Shift-click (to select several adjacent files)
or Ctrl-click (to select non-adjacent files).
Pageant will now load the private key(s). If a key is protected by a
passphrase, Pageant will ask you to type the passphrase.
(This is not the only way to add a private key to Pageant. You can
also add one from a remote system by using agent forwarding; see
section 9.4 for details.)
9.2.3 The `Remove Key' button
If you need to remove a key from Pageant, select that key in the
list box, and press the `Remove Key' button. Pageant will remove the
key from its memory.
You can apply this to keys you added using the `Add Key' button, or
to keys you added remotely using agent forwarding (see section 9.4);
it makes no difference.
9.3 The Pageant command line
Pageant can be made to do things automatically when it starts up,
by specifying instructions on its command line. If you're starting
Pageant from the Windows GUI, you can arrange this by editing the
properties of the Windows shortcut that it was started from.
9.3.1 Making Pageant automatically load keys on startup
Pageant can automatically load one or more private keys when it
starts up, if you provide them on the Pageant command line. Your
command line might then look like:
C:\PuTTY\pageant.exe d:\main.ppk d:\secondary.ppk
If the keys are stored encrypted, Pageant will request the
passphrases on startup.
9.3.2 Making Pageant run another program
You can arrange for Pageant to start another program once it has
initialised itself and loaded any keys specified on its command
line. This program (perhaps a PuTTY, or a WinCVS making use of
Plink, or whatever) will then be able to use the keys Pageant has
loaded.
You do this by specifying the `-c' option followed by the command,
like this:
C:\PuTTY\pageant.exe d:\main.ppk -c C:\PuTTY\putty.exe
9.4 Using agent forwarding
Agent forwarding is a mechanism that allows applications on your SSH
server machine to talk to the agent on your client machine.
Note that at present, agent forwarding in SSH-2 is only available
when your SSH server is OpenSSH. The ssh.com server uses a different
agent protocol, which PuTTY does not yet support.
To enable agent forwarding, first start Pageant. Then set up a
PuTTY SSH session in which `Allow agent forwarding' is enabled (see
section 4.20.3). Open the session as normal. (Alternatively, you can
use the `-A' command line option; see section 3.7.3.9 for details.)
If this has worked, your applications on the server should now have
access to a Unix domain socket which the SSH server will forward
back to PuTTY, and PuTTY will forward on to the agent. To check that
this has actually happened, you can try this command on Unix server
machines:
unixbox:~$ echo $SSH_AUTH_SOCK
/tmp/ssh-XXNP18Jz/agent.28794
unixbox:~$
If the result line comes up blank, agent forwarding has not been
enabled at all.
Now if you run `ssh' on the server and use it to connect through to
another server that accepts one of the keys in Pageant, you should
be able to log in without a password:
unixbox:~$ ssh -v otherunixbox
[...]
debug: next auth method to try is publickey
debug: userauth_pubkey_agent: trying agent key my-putty-key
debug: ssh-userauth2 successful: method publickey
[...]
If you enable agent forwarding on _that_ SSH connection as well (see
the manual for your server-side SSH client to find out how to do
this), your authentication keys will still be available on the next
machine you connect to - two SSH connections away from where they're
actually stored.
In addition, if you have a private key on one of the SSH servers,
you can send it all the way back to Pageant using the local `ssh-
add' command:
unixbox:~$ ssh-add ~/.ssh/id_rsa
Need passphrase for /home/fred/.ssh/id_rsa
Enter passphrase for /home/fred/.ssh/id_rsa:
Identity added: /home/fred/.ssh/id_rsa (/home/simon/.ssh/id_rsa)
unixbox:~$
and then it's available to every machine that has agent forwarding
available (not just the ones downstream of the place you added it).
9.5 Security considerations
Using Pageant for public-key authentication gives you the
convenience of being able to open multiple SSH sessions without
having to type a passphrase every time, but also gives you the
security benefit of never storing a decrypted private key on disk.
Many people feel this is a good compromise between security and
convenience.
It _is_ a compromise, however. Holding your decrypted private keys
in Pageant is better than storing them in easy-to-find disk files,
but still less secure than not storing them anywhere at all. This is
for two reasons:
- Windows unfortunately provides no way to protect pieces of
memory from being written to the system swap file. So if Pageant
is holding your private keys for a long period of time, it's
possible that decrypted private key data may be written to the
system swap file, and an attacker who gained access to your hard
disk later on might be able to recover that data. (However,
if you stored an unencrypted key in a disk file they would
_certainly_ be able to recover it.)
- Although, like most modern operating systems, Windows prevents
programs from accidentally accessing one another's memory space,
it does allow programs to access one another's memory space
deliberately, for special purposes such as debugging. This means
that if you allow a virus, trojan, or other malicious program
on to your Windows system while Pageant is running, it could
access the memory of the Pageant process, extract your decrypted
authentication keys, and send them back to its master.
Similarly, use of agent _forwarding_ is a security improvement on
other methods of one-touch authentication, but not perfect. Holding
your keys in Pageant on your Windows box has a security advantage
over holding them on the remote server machine itself (either in an
agent or just unencrypted on disk), because if the server machine
ever sees your unencrypted private key then the sysadmin or anyone
who cracks the machine can steal the keys and pretend to be you for
as long as they want.
However, the sysadmin of the server machine can always pretend to
be you _on that machine_. So if you forward your agent to a server
machine, then the sysadmin of that machine can access the forwarded
agent connection and request signatures from your public keys, and
can therefore log in to other machines as you. They can only do this
to a limited extent - when the agent forwarding disappears they
lose the ability - but using Pageant doesn't actually _prevent_ the
sysadmin (or hackers) on the server from doing this.
Therefore, if you don't trust the sysadmin of a server machine, you
should _never_ use agent forwarding to that machine. (Of course you
also shouldn't store private keys on that machine, type passphrases
into it, or log into other machines from it in any way at all;
Pageant is hardly unique in this respect.)
Chapter 10: Common error messages
---------------------------------
This chapter lists a number of common error messages which PuTTY and
its associated tools can produce, and explains what they mean in
more detail.
We do not attempt to list _all_ error messages here: there are many
which should never occur, and some which should be self-explanatory.
If you get an error message which is not listed in this chapter and
which you don't understand, report it to us as a bug (see appendix
B) and we will add documentation for it.
10.1 `The server's host key is not cached in the registry'
This error message occurs when PuTTY connects to a new SSH server.
Every server identifies itself by means of a host key; once PuTTY
knows the host key for a server, it will be able to detect if a
malicious attacker redirects your connection to another machine.
If you see this message, it means that PuTTY has not seen this host
key before, and has no way of knowing whether it is correct or not.
You should attempt to verify the host key by other means, such as
asking the machine's administrator.
If you see this message and you know that your installation of PuTTY
_has_ connected to the same server before, it may have been recently
upgraded to SSH protocol version 2. SSH protocols 1 and 2 use
separate host keys, so when you first use SSH-2 with a server you
have only used SSH-1 with before, you will see this message again.
You should verify the correctness of the key as before.
See section 2.2 for more information on host keys.
10.2 `WARNING - POTENTIAL SECURITY BREACH!'
This message, followed by `The server's host key does not match
the one PuTTY has cached in the registry', means that PuTTY has
connected to the SSH server before, knows what its host key _should_
be, but has found a different one.
This may mean that a malicious attacker has replaced your server
with a different one, or has redirected your network connection
to their own machine. On the other hand, it may simply mean that
the administrator of your server has accidentally changed the key
while upgrading the SSH software; this _shouldn't_ happen but it is
unfortunately possible.
You should contact your server's administrator and see whether they
expect the host key to have changed. If so, verify the new host key
in the same way as you would if it was new.
See section 2.2 for more information on host keys.
10.3 `Out of space for port forwardings'
PuTTY has a fixed-size buffer which it uses to store the details
of all port forwardings you have set up in an SSH session. If you
specify too many port forwardings on the PuTTY or Plink command line
and this buffer becomes full, you will see this error message.
We need to fix this (fixed-size buffers are almost always a mistake)
but we haven't got round to it. If you actually have trouble with
this, let us know and we'll move it up our priority list.
10.4 `The first cipher supported by the server is ... below the
configured warning threshold'
This occurs when the SSH server does not offer any ciphers which you
have configured PuTTY to consider strong enough. By default, PuTTY
puts up this warning only for single-DES encryption.
See section 4.18.6 for more information on this message.
10.5 `Server sent disconnect message type 2
(SSH_DISCONNECT_PROTOCOL_ERROR): "Too many authentication failures
for root"'
This message is produced by an OpenSSH (or Sun SSH) server if it
receives more failed authentication attempts than it is willing to
tolerate. This can easily happen if you are using Pageant and have
a large number of keys loaded into it. This can be worked around
on the server by disabling public-key authentication or (for Sun
SSH only) by increasing `MaxAuthTries' in `sshd_config'. Neither of
these is a really satisfactory solution, and we hope to provide a
better one in a future version of PuTTY.
10.6 `Out of memory'
This occurs when PuTTY tries to allocate more memory than the system
can give it. This _may_ happen for genuine reasons: if the computer
really has run out of memory, or if you have configured an extremely
large number of lines of scrollback in your terminal. PuTTY is
not able to recover from running out of memory; it will terminate
immediately after giving this error.
However, this error can also occur when memory is not running out at
all, because PuTTY receives data in the wrong format. In SSH-2 and
also in SFTP, the server sends the length of each message before the
message itself; so PuTTY will receive the length, try to allocate
space for the message, and then receive the rest of the message.
If the length PuTTY receives is garbage, it will try to allocate
a ridiculous amount of memory, and will terminate with an `Out of
memory' error.
This can happen in SSH-2, if PuTTY and the server have not enabled
encryption in the same way (see question A.7.5 in the FAQ). Some
versions of OpenSSH have a known problem with this: see question
A.7.16.
This can also happen in PSCP or PSFTP, if your login scripts on the
server generate output: the client program will be expecting an
SFTP message starting with a length, and if it receives some text
from your login scripts instead it will try to interpret them as a
message length. See question A.7.6 for details of this.
10.7 `Internal error', `Internal fault', `Assertion failed'
Any error beginning with the word `Internal' should _never_ occur.
If it does, there is a bug in PuTTY by definition; please see
appendix B and report it to us.
Similarly, any error message starting with `Assertion failed' is a
bug in PuTTY. Please report it to us, and include the exact text
from the error message box.
10.8 `Unable to use this private key file', `Couldn't load private key',
`Key is of wrong type'
Various forms of this error are printed in the PuTTY window, or
written to the PuTTY Event Log (see section 3.1.3.1) when trying
public-key authentication, or given by Pageant when trying to load a
private key.
If you see one of these messages, it often indicates that you've
tried to load a key of an inappropriate type into PuTTY, Plink,
PSCP, PSFTP, or Pageant.
You may have specified a key that's inappropriate for the connection
you're making. The SSH-1 and SSH-2 protocols require different
private key formats, and a SSH-1 key can't be used for a SSH-2
connection (or vice versa).
Alternatively, you may have tried to load an SSH-2 key in a
`foreign' format (OpenSSH or ssh.com) directly into one of the PuTTY
tools, in which case you need to import it into PuTTY's native
format (`*.PPK') using PuTTYgen - see section 8.2.12.
10.9 `Server refused our public key' or `Key refused'
Various forms of this error are printed in the PuTTY window, or
written to the PuTTY Event Log (see section 3.1.3.1) when trying
public-key authentication.
If you see one of these messages, it means that PuTTY has sent a
public key to the server and offered to authenticate with it, and
the server has refused to accept authentication. This usually means
that the server is not configured to accept this key to authenticate
this user.
This is almost certainly not a problem with PuTTY. If you see
this type of message, the first thing you should do is check your
_server_ configuration carefully. Common errors include having the
wrong permissions or ownership set on the public key or the user's
home directory on the server. Also, read the PuTTY Event Log; the
server may have sent diagnostic messages explaining exactly what
problem it had with your setup.
10.10 `Access denied', `Authentication refused'
Various forms of this error are printed in the PuTTY window,
or written to the PuTTY Event Log (see section 3.1.3.1) during
authentication.
If you see one of these messages, it means that the server has
refused all the forms of authentication PuTTY has tried and it has
no further ideas.
It may be worth checking the Event Log for diagnostic messages from
the server giving more detail.
This error can be caused by buggy SSH-1 servers that fail to cope
with the various strategies we use for camouflaging passwords in
transit. Upgrade your server, or use the workarounds described in
section 4.23.1 and possibly section 4.23.2.
10.11 `Incorrect CRC received on packet' or `Incorrect MAC received on
packet'
This error occurs when PuTTY decrypts an SSH packet and its checksum
is not correct. This probably means something has gone wrong in the
encryption or decryption process. It's difficult to tell from this
error message whether the problem is in the client, in the server,
or in between.
A known server problem which can cause this error is described in
question A.7.16 in the FAQ.
10.12 `Incoming packet was garbled on decryption'
This error occurs when PuTTY decrypts an SSH packet and the
decrypted data makes no sense. This probably means something has
gone wrong in the encryption or decryption process. It's difficult
to tell from this error message whether the problem is in the
client, in the server, or in between.
If you get this error, one thing you could try would be to fiddle
with the setting of `Miscomputes SSH-2 encryption keys' on the Bugs
panel (see section 4.23.5).
Another known server problem which can cause this error is described
in question A.7.16 in the FAQ.
10.13 `PuTTY X11 proxy: _various errors_'
This family of errors are reported when PuTTY is doing X forwarding.
They are sent back to the X application running on the SSH server,
which will usually report the error to the user.
When PuTTY enables X forwarding (see section 3.4) it creates a
virtual X display running on the SSH server. This display requires
authentication to connect to it (this is how PuTTY prevents other
users on your server machine from connecting through the PuTTY proxy
to your real X display). PuTTY also sends the server the details it
needs to enable clients to connect, and the server should put this
mechanism in place automatically, so your X applications should just
work.
A common reason why people see one of these messages is because they
used SSH to log in as one user (let's say `fred'), and then used
the Unix `su' command to become another user (typically `root').
The original user, `fred', has access to the X authentication data
provided by the SSH server, and can run X applications which are
forwarded over the SSH connection. However, the second user (`root')
does not automatically have the authentication data passed on to it,
so attempting to run an X application as that user often fails with
this error.
If this happens, _it is not a problem with PuTTY_. You need to
arrange for your X authentication data to be passed from the user
you logged in as to the user you used `su' to become. How you do
this depends on your particular system; in fact many modern versions
of `su' do it automatically.
10.14 `Network error: Software caused connection abort'
This is a generic error produced by the Windows network code when
it kills an established connection for some reason. For example, it
might happen if you pull the network cable out of the back of an
Ethernet-connected computer, or if Windows has any other similar
reason to believe the entire network has become unreachable.
Windows also generates this error if it has given up on the machine
at the other end of the connection ever responding to it. If the
network between your client and server goes down and your client
then tries to send some data, Windows will make several attempts
to send the data and will then give up and kill the connection. In
particular, this can occur even if you didn't type anything, if you
are using SSH-2 and PuTTY attempts a key re-exchange. (See section
4.19.2 for more about key re-exchange.)
(It can also occur if you are using keepalives in your connection.
Other people have reported that keepalives _fix_ this error for
them. See section 4.13.1 for a discussion of the pros and cons of
keepalives.)
We are not aware of any reason why this error might occur that would
represent a bug in PuTTY. The problem is between you, your Windows
system, your network and the remote system.
10.15 `Network error: Connection reset by peer'
This error occurs when the machines at each end of a network
connection lose track of the state of the connection between them.
For example, you might see it if your SSH server crashes, and
manages to reboot fully before you next attempt to send data to it.
However, the most common reason to see this message is if you are
connecting through a firewall or a NAT router which has timed the
connection out. See question A.7.10 in the FAQ for more details.
You may be able to improve the situation by using keepalives; see
section 4.13.1 for details on this.
Note that Windows can produce this error in some circumstances
without seeing a connection reset from the server, for instance if
the connection to the network is lost.
10.16 `Network error: Connection refused'
This error means that the network connection PuTTY tried to make to
your server was rejected by the server. Usually this happens because
the server does not provide the service which PuTTY is trying to
access.
Check that you are connecting with the correct protocol (SSH, Telnet
or Rlogin), and check that the port number is correct. If that
fails, consult the administrator of your server.
10.17 `Network error: Connection timed out'
This error means that the network connection PuTTY tried to make to
your server received no response at all from the server. Usually
this happens because the server machine is completely isolated from
the network, or because it is turned off.
Check that you have correctly entered the host name or IP address
of your server machine. If that fails, consult the administrator of
your server.
Unix also generates this error when it tries to send data down a
connection and contact with the server has been completely lost
during a connection. (There is a delay of minutes before Unix gives
up on receiving a reply from the server.) This can occur if you type
things into PuTTY while the network is down, but it can also occur
if PuTTY decides of its own accord to send data: due to a repeat key
exchange in SSH-2 (see section 4.19.2) or due to keepalives (section
4.13.1).
Appendix A: PuTTY FAQ
---------------------
This FAQ is published on the PuTTY web site, and also provided as an
appendix in the manual.
A.1 Introduction
A.1.1 What is PuTTY?
PuTTY is a client program for the SSH, Telnet and Rlogin network
protocols.
These protocols are all used to run a remote session on a computer,
over a network. PuTTY implements the client end of that session: the
end at which the session is displayed, rather than the end at which
it runs.
In really simple terms: you run PuTTY on a Windows machine, and
tell it to connect to (for example) a Unix machine. PuTTY opens a
window. Then, anything you type into that window is sent straight
to the Unix machine, and everything the Unix machine sends back is
displayed in the window. So you can work on the Unix machine as if
you were sitting at its console, while actually sitting somewhere
else.
A.2 Features supported in PuTTY
In general, if you want to know if PuTTY supports a particular
feature, you should look for it on the PuTTY web site. In
particular:
- try the changes page, and see if you can find the feature on
there. If a feature is listed there, it's been implemented. If
it's listed as a change made _since_ the latest version, it
should be available in the development snapshots, in which case
testing will be very welcome.
- try the Wishlist page, and see if you can find the feature
there. If it's on there, and not in the `Recently fixed'
section, it probably _hasn't_ been implemented.
A.2.1 Does PuTTY support SSH-2?
Yes. SSH-2 support has been available in PuTTY since version 0.50.
Public key authentication (both RSA and DSA) in SSH-2 is new in
version 0.52.
A.2.2 Does PuTTY support reading OpenSSH or ssh.com SSH-2 private key
files?
PuTTY doesn't support this natively, but as of 0.53 PuTTYgen can
convert both OpenSSH and ssh.com private key files into PuTTY's
format.
A.2.3 Does PuTTY support SSH-1?
Yes. SSH-1 support has always been available in PuTTY.
A.2.4 Does PuTTY support local echo?
Yes. Version 0.52 has proper support for local echo.
In version 0.51 and before, local echo could not be separated from
local line editing (where you type a line of text locally, and it
is not sent to the server until you press Return, so you have the
chance to edit it and correct mistakes _before_ the server sees it).
New in version 0.52, local echo and local line editing are separate
options, and by default PuTTY will try to determine automatically
whether to enable them or not, based on which protocol you have
selected and also based on hints from the server. If you have a
problem with PuTTY's default choice, you can force each option to be
enabled or disabled as you choose. The controls are in the Terminal
panel, in the section marked `Line discipline options'.
A.2.5 Does PuTTY support storing settings, so I don't have to change them
every time?
Yes, all of PuTTY's settings can be saved in named session profiles.
You can also change the default settings that are used for new
sessions. See section 4.1.2 in the documentation for how to do this.
A.2.6 Does PuTTY support storing its settings in a disk file?
Not at present, although section 4.24 in the documentation gives a
method of achieving the same effect.
A.2.7 Does PuTTY support full-screen mode, like a DOS box?
Yes; this is a new feature in version 0.52.
A.2.8 Does PuTTY have the ability to remember my password so I don't have
to type it every time?
No, it doesn't.
Remembering your password is a bad plan for obvious security
reasons: anyone who gains access to your machine while you're away
from your desk can find out the remembered password, and use it,
abuse it or change it.
In addition, it's not even _possible_ for PuTTY to automatically
send your password in a Telnet session, because Telnet doesn't
give the client software any indication of which part of the login
process is the password prompt. PuTTY would have to guess, by
looking for words like `password' in the session data; and if your
login program is written in something other than English, this won't
work.
In SSH, remembering your password would be possible in theory,
but there doesn't seem to be much point since SSH supports public
key authentication, which is more flexible and more secure. See
chapter 8 in the documentation for a full discussion of public key
authentication.
A.2.9 Is there an option to turn off the annoying host key prompts?
No, there isn't. And there won't be. Even if you write it yourself
and send us the patch, we won't accept it.
Those annoying host key prompts are the _whole point_ of SSH.
Without them, all the cryptographic technology SSH uses to secure
your session is doing nothing more than making an attacker's job
slightly harder; instead of sitting between you and the server with
a packet sniffer, the attacker must actually subvert a router and
start modifying the packets going back and forth. But that's not all
that much harder than just sniffing; and without host key checking,
it will go completely undetected by client or server.
Host key checking is your guarantee that the encryption you put
on your data at the client end is the _same_ encryption taken off
the data at the server end; it's your guarantee that it hasn't
been removed and replaced somewhere on the way. Host key checking
makes the attacker's job _astronomically_ hard, compared to packet
sniffing, and even compared to subverting a router. Instead of
applying a little intelligence and keeping an eye on Bugtraq, the
attacker must now perform a brute-force attack against at least one
military-strength cipher. That insignificant host key prompt really
does make _that_ much difference.
If you're having a specific problem with host key checking - perhaps
you want an automated batch job to make use of PSCP or Plink, and
the interactive host key prompt is hanging the batch process -
then the right way to fix it is to add the correct host key to the
Registry in advance. That way, you retain the _important_ feature of
host key checking: the right key will be accepted and the wrong ones
will not. Adding an option to turn host key checking off completely
is the wrong solution and we will not do it.
If you have host keys available in the common `known_hosts' format,
we have a script called `kh2reg.py' to convert them to a Windows
.REG file, which can be installed ahead of time by double-clicking
or using `REGEDIT'.
A.2.10 Will you write an SSH server for the PuTTY suite, to go with the
client?
No. The only reason we might want to would be if we could easily re-
use existing code and significantly cut down the effort. We don't
believe this is the case; there just isn't enough common ground
between an SSH client and server to make it worthwhile.
If someone else wants to use bits of PuTTY in the process of writing
a Windows SSH server, they'd be perfectly welcome to of course, but
I really can't see it being a lot less effort for us to do that than
it would be for us to write a server from the ground up. We don't
have time, and we don't have motivation. The code is available if
anyone else wants to try it.
A.2.11 Can PSCP or PSFTP transfer files in ASCII mode?
Unfortunately not.
Until recently, this was a limitation of the file transfer
protocols: the SCP and SFTP protocols had no notion of transferring
a file in anything other than binary mode. (This is still true of
SCP.)
The current draft protocol spec of SFTP proposes a means of
implementing ASCII transfer. At some point PSCP/PSFTP may implement
this proposal.
A.3 Ports to other operating systems
The eventual goal is for PuTTY to be a multi-platform program, able
to run on at least Windows, Mac OS and Unix.
Porting will become easier once PuTTY has a generalised porting
layer, drawing a clear line between platform-dependent and platform-
independent code. The general intention was for this porting layer
to evolve naturally as part of the process of doing the first port;
a Unix port has now been released and the plan seems to be working
so far.
A.3.1 What ports of PuTTY exist?
Currently, release versions of PuTTY tools only run on full Win32
systems and Unix. `Win32' includes Windows 95, 98, and ME, and it
includes Windows NT, Windows 2000 and Windows XP.
In the development code, a partial port to the Mac OS (see question
A.3.6) is under way.
Currently PuTTY does _not_ run on Windows CE (see question A.3.4),
and it does not quite run on the Win32s environment under Windows
3.1 (see question A.3.5).
We do not have release-quality ports for any other systems at the
present time. If anyone told you we had an EPOC port, or an iPaq
port, or any other port of PuTTY, they were mistaken. We don't.
There are some third-party ports to various platforms, mentioned on
the Links page of our website.
A.3.2 Is there a port to Unix?
As of 0.54, there are Unix ports of most of the traditional PuTTY
tools, and also one entirely new application.
If you look at the source release, you should find a `unix'
subdirectory containing `Makefile.gtk', which should build you
Unix ports of Plink, PuTTY itself, PuTTYgen, PSCP, PSFTP, and also
`pterm' - an xterm-type program which supports the same terminal
emulation as PuTTY. We do not yet have a Unix port of Pageant.
If you don't have Gtk, you should still be able to build the
command-line tools.
Note that Unix PuTTY has mostly only been tested on Linux so far;
portability problems such as BSD-style ptys or different header file
requirements are expected.
A.3.3 What's the point of the Unix port? Unix has OpenSSH.
All sorts of little things. `pterm' is directly useful to anyone who
prefers PuTTY's terminal emulation to `xterm''s, which at least some
people do. Unix Plink has apparently found a niche among people who
find the complexity of OpenSSL makes OpenSSH hard to install (and
who don't mind Plink not having as many features). Some users want
to generate a large number of SSH keys on Unix and then copy them
all into PuTTY, and the Unix PuTTYgen should allow them to automate
that conversion process.
There were development advantages as well; porting PuTTY to Unix
was a valuable path-finding effort for other future ports, and also
allowed us to use the excellent Linux tool Valgrind to help with
debugging, which has already improved PuTTY's stability on _all_
platforms.
However, if you're a Unix user and you can see no reason to switch
from OpenSSH to PuTTY/Plink, then you're probably right. We don't
expect our Unix port to be the right thing for everybody.
A.3.4 Will there be a port to Windows CE or PocketPC?
It's currently being worked on, but it's only in its early stages
yet, and certainly isn't yet useful. PuTTY on portable devices would
clearly be a useful thing, so in the long term I hope it can be
brought up to release quality.
There's also a third-party port at `http://pocketputty.duxy.net/'.
A.3.5 Is there a port to Windows 3.1?
PuTTY is a 32-bit application from the ground up, so it won't run on
Windows 3.1 as a native 16-bit program; and it would be _very_ hard
to port it to do so, because of Windows 3.1's vile memory allocation
mechanisms.
However, it is possible in theory to compile the existing PuTTY
source in such a way that it will run under Win32s (an extension to
Windows 3.1 to let you run 32-bit programs). In order to do this
you'll need the right kind of C compiler - modern versions of Visual
C at least have stopped being backwards compatible to Win32s. Also,
the last time we tried this it didn't work very well.
If you're interested in running PuTTY under Windows 3.1, help and
testing in this area would be very welcome!
A.3.6 Will there be a port to the Mac?
There are several answers to this question:
- The Unix/Gtk port is already fully working under Mac OS X as an
X11 application.
- A native (Cocoa) Mac OS X port is in progress. It's just about
usable, but is of nowhere near release quality yet, and is
likely to behave in unexpected ways.
- A separate port to the classic Mac OS (pre-OSX) is also in
progress; it too is not ready yet.
A.3.7 Will there be a port to EPOC?
I hope so, but given that ports aren't really progressing very fast
even on systems the developers _do_ already know how to program for,
it might be a long time before any of us get round to learning a new
system and doing the port for that.
However, some of the work has been done by other people, and a beta
port of PuTTY for the Nokia 9200 Communicator series is available
from http://s2putty.sourceforge.net/
A.4 Embedding PuTTY in other programs
A.4.1 Is the SSH or Telnet code available as a DLL?
No, it isn't. It would take a reasonable amount of rewriting for
this to be possible, and since the PuTTY project itself doesn't
believe in DLLs (they make installation more error-prone) none of us
has taken the time to do it.
Most of the code cleanup work would be a good thing to happen in
general, so if anyone feels like helping, we wouldn't say no.
A.4.2 Is the SSH or Telnet code available as a Visual Basic component?
No, it isn't. None of the PuTTY team uses Visual Basic, and none of
us has any particular need to make SSH connections from a Visual
Basic application. In addition, all the preliminary work to turn it
into a DLL would be necessary first; and furthermore, we don't even
know how to write VB components.
If someone offers to do some of this work for us, we might consider
it, but unless that happens I can't see VB integration being
anywhere other than the very bottom of our priority list.
A.4.3 How can I use PuTTY to make an SSH connection from within another
program?
Probably your best bet is to use Plink, the command-line connection
tool. If you can start Plink as a second Windows process, and
arrange for your primary process to be able to send data to the
Plink process, and receive data from it, through pipes, then you
should be able to make SSH connections from your program.
This is what CVS for Windows does, for example.
A.5 Details of PuTTY's operation
A.5.1 What terminal type does PuTTY use?
For most purposes, PuTTY can be considered to be an xterm terminal.
PuTTY also supports some terminal control sequences not supported by
the real xterm: notably the Linux console sequences that reconfigure
the colour palette, and the title bar control sequences used by
DECterm (which are different from the xterm ones; PuTTY supports
both).
By default, PuTTY announces its terminal type to the server as
`xterm'. If you have a problem with this, you can reconfigure it to
say something else; `vt220' might help if you have trouble.
A.5.2 Where does PuTTY store its data?
On Windows, PuTTY stores most of its data (saved sessions, SSH host
keys) in the Registry. The precise location is
HKEY_CURRENT_USER\Software\SimonTatham\PuTTY
and within that area, saved sessions are stored under `Sessions'
while host keys are stored under `SshHostKeys'.
PuTTY also requires a random number seed file, to improve the
unpredictability of randomly chosen data needed as part of the SSH
cryptography. This is stored by default in a file called `PUTTY.RND'
in your Windows home directory (`%HOMEDRIVE%\%HOMEPATH%'), or in
the actual Windows directory (such as `C:\WINDOWS') if the home
directory doesn't exist, for example if you're using Win95. If you
want to change the location of the random number seed file, you can
put your chosen pathname in the Registry, at
HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\RandSeedFile
On Unix, PuTTY stores all of this data in a directory ~/.putty.
A.6 HOWTO questions
A.6.1 What commands can I type into my PuTTY terminal window?
This is not a question you should be asking _us_. You need to read
the manuals, or ask the administrator, of _the computer you have
connected to_.
PuTTY does not process the commands you type into it. It's only a
communications tool. It makes a connection to another computer; it
passes the commands you type to that other computer; and it passes
the other computer's responses back to you. Therefore, the precise
range of commands you can use will not depend on PuTTY, but on what
kind of computer you have connected to and what software is running
on it. The PuTTY team cannot help you with that.
(Think of PuTTY as being a bit like a telephone. If you phone
somebody up and you don't know what language to speak to make them
understand you, it isn't _the telephone company_'s job to find that
out for you. We just provide the means for you to get in touch;
making yourself understood is somebody else's problem.)
If you are unsure of where to start looking for the administrator
of your server, a good place to start might be to remember how you
found out the host name in the PuTTY configuration. If you were
given that host name by e-mail, for example, you could try asking
the person who sent you that e-mail. If your company's IT department
provided you with ready-made PuTTY saved sessions, then that IT
department can probably also tell you something about what commands
you can type during those sessions. But the PuTTY maintainer team
does not administer any server you are likely to be connecting to,
and cannot help you with questions of this type.
A.6.2 How can I make PuTTY start up maximised?
Create a Windows shortcut to start PuTTY from, and set it as `Run
Maximized'.
A.6.3 How can I create a Windows shortcut to start a particular saved
session directly?
To run a PuTTY session saved under the name `mysession', create a
Windows shortcut that invokes PuTTY with a command line like
\path\name\to\putty.exe -load mysession
(Note: prior to 0.53, the syntax was `@session'. This is now
deprecated and may be removed at some point.)
A.6.4 How can I start an SSH session straight from the command line?
Use the command line `putty -ssh host.name'. Alternatively, create a
saved session that specifies the SSH protocol, and start the saved
session as shown in question A.6.3.
A.6.5 How do I copy and paste between PuTTY and other Windows
applications?
Copy and paste works similarly to the X Window System. You use the
left mouse button to select text in the PuTTY window. The act of
selection _automatically_ copies the text to the clipboard: there
is no need to press Ctrl-Ins or Ctrl-C or anything else. In fact,
pressing Ctrl-C will send a Ctrl-C character to the other end of
your connection (just like it does the rest of the time), which may
have unpleasant effects. The _only_ thing you need to do, to copy
text to the clipboard, is to select it.
To paste the clipboard contents into a PuTTY window, by default you
click the right mouse button. If you have a three-button mouse and
are used to X applications, you can configure pasting to be done by
the middle button instead, but this is not the default because most
Windows users don't have a middle button at all.
You can also paste by pressing Shift-Ins.
A.6.6 How do I use all PuTTY's features (public keys, proxying, cipher
selection, etc.) in PSCP, PSFTP and Plink?
Most major features (e.g., public keys, port forwarding) are
available through command line options. See the documentation.
Not all features are accessible from the command line yet, although
we'd like to fix this. In the meantime, you can use most of PuTTY's
features if you create a PuTTY saved session, and then use the name
of the saved session on the command line in place of a hostname.
This works for PSCP, PSFTP and Plink (but don't expect port
forwarding in the file transfer applications!).
A.6.7 How do I use PSCP.EXE? When I double-click it gives me a command
prompt window which then closes instantly.
PSCP is a command-line application, not a GUI application. If you
run it without arguments, it will simply print a help message and
terminate.
To use PSCP properly, run it from a Command Prompt window. See
chapter 5 in the documentation for more details.
A.6.8 How do I use PSCP to copy a file whose name has spaces in?
If PSCP is using the traditional SCP protocol, this is confusing. If
you're specifying a file at the local end, you just use one set of
quotes as you would normally do:
pscp "local filename with spaces" user@host:
pscp user@host:myfile "local filename with spaces"
But if the filename you're specifying is on the _remote_ side, you
have to use backslashes and two sets of quotes:
pscp user@host:"\"remote filename with spaces\"" local_filename
pscp local_filename user@host:"\"remote filename with spaces\""
Worse still, in a remote-to-local copy you have to specify the local
file name explicitly, otherwise PSCP will complain that they don't
match (unless you specified the `-unsafe' option). The following
command will give an error message:
c:\>pscp user@host:"\"oo er\"" .
warning: remote host tried to write to a file called 'oo er'
when we requested a file called '"oo er"'.
Instead, you need to specify the local file name in full:
c:\>pscp user@host:"\"oo er\"" "oo er"
If PSCP is using the newer SFTP protocol, none of this is a problem,
and all filenames with spaces in are specified using a single pair
of quotes in the obvious way:
pscp "local file" user@host:
pscp user@host:"remote file" .
A.7 Troubleshooting
A.7.1 Why do I see `Incorrect MAC received on packet'?
One possible cause of this that used to be common is a bug in old
SSH-2 servers distributed by ssh.com. (This is not the only possible
cause; see section 10.11 in the documentation.) Version 2.3.0 and
below of their SSH-2 server constructs Message Authentication Codes
in the wrong way, and expects the client to construct them in the
same wrong way. PuTTY constructs the MACs correctly by default, and
hence these old servers will fail to work with it.
If you are using PuTTY version 0.52 or better, this should work
automatically: PuTTY should detect the buggy servers from their
version number announcement, and automatically start to construct
its MACs in the same incorrect manner as they do, so it will be able
to work with them.
If you are using PuTTY version 0.51 or below, you can enable the
workaround by going to the SSH panel and ticking the box labelled
`Imitate SSH2 MAC bug'. It's possible that you might have to do this
with 0.52 as well, if a buggy server exists that PuTTY doesn't know
about.
In this context MAC stands for Message Authentication Code. It's a
cryptographic term, and it has nothing at all to do with Ethernet
MAC (Media Access Control) addresses.
A.7.2 Why do I see `Fatal: Protocol error: Expected control record' in
PSCP?
This happens because PSCP was expecting to see data from the server
that was part of the PSCP protocol exchange, and instead it saw data
that it couldn't make any sense of at all.
This almost always happens because the startup scripts in your
account on the server machine are generating output. This is
impossible for PSCP, or any other SCP client, to work around. You
should never use startup files (`.bashrc', `.cshrc' and so on) which
generate output in non-interactive sessions.
This is not actually a PuTTY problem. If PSCP fails in this way,
then all other SCP clients are likely to fail in exactly the same
way. The problem is at the server end.
A.7.3 I clicked on a colour in the Colours panel, and the colour didn't
change in my terminal.
That isn't how you're supposed to use the Colours panel.
During the course of a session, PuTTY potentially uses _all_ the
colours listed in the Colours panel. It's not a question of using
only one of them and you choosing which one; PuTTY will use them
_all_. The purpose of the Colours panel is to let you adjust the
appearance of all the colours. So to change the colour of the
cursor, for example, you would select `Cursor Colour', press the
`Modify' button, and select a new colour from the dialog box that
appeared. Similarly, if you want your session to appear in green,
you should select `Default Foreground' and press `Modify'. Clicking
on `ANSI Green' won't turn your session green; it will only allow
you to adjust the _shade_ of green used when PuTTY is instructed by
the server to display green text.
A.7.4 Plink on Windows 95 says it can't find WS2_32.DLL.
Plink requires the extended Windows network library, WinSock version
2. This is installed as standard on Windows 98 and above, and on
Windows NT, and even on later versions of Windows 95; but early
Win95 installations don't have it.
In order to use Plink on these systems, you will need to download
the WinSock 2 upgrade:
http://www.microsoft.com/windows95/downloads/contents/
wuadmintools/s_wunetworkingtools/w95sockets2/
A.7.5 After trying to establish an SSH-2 connection, PuTTY says `Out of
memory' and dies.
If this happens just while the connection is starting up, this often
indicates that for some reason the client and server have failed to
establish a session encryption key. Somehow, they have performed
calculations that should have given each of them the same key, but
have ended up with different keys; so data encrypted by one and
decrypted by the other looks like random garbage.
This causes an `out of memory' error because the first encrypted
data PuTTY expects to see is the length of an SSH message. Normally
this will be something well under 100 bytes. If the decryption has
failed, PuTTY will see a completely random length in the region of
two _gigabytes_, and will try to allocate enough memory to store
this non-existent message. This will immediately lead to it thinking
it doesn't have enough memory, and panicking.
If this happens to you, it is quite likely to still be a PuTTY bug
and you should report it (although it might be a bug in your SSH
server instead); but it doesn't necessarily mean you've actually run
out of memory.
A.7.6 When attempting a file transfer, either PSCP or PSFTP says `Out of
memory' and dies.
This is almost always caused by your login scripts on the server
generating output. PSCP or PSFTP will receive that output when they
were expecting to see the start of a file transfer protocol, and
they will attempt to interpret the output as file-transfer protocol.
This will usually lead to an `out of memory' error for much the same
reasons as given in question A.7.5.
This is a setup problem in your account on your server, _not_ a
PSCP/PSFTP bug. Your login scripts should _never_ generate output
during non-interactive sessions; secure file transfer is not the
only form of remote access that will break if they do.
On Unix, a simple fix is to ensure that all the parts of your login
script that might generate output are in `.profile' (if you use a
Bourne shell derivative) or `.login' (if you use a C shell). Putting
them in more general files such as `.bashrc' or `.cshrc' is liable
to lead to problems.
A.7.7 PSFTP transfers files much slower than PSCP.
The throughput of PSFTP 0.54 should be much better than 0.53b and
prior; we've added code to the SFTP backend to queue several blocks
of data rather than waiting for an acknowledgement for each. (The
SCP backend did not suffer from this performance issue because SCP
is a much simpler protocol.)
A.7.8 When I run full-colour applications, I see areas of black space
where colour ought to be, or vice versa.
You almost certainly need to change the `Use background colour to
erase screen' setting in the Terminal panel. If there is too much
black space (the commoner situation), you should enable it, while
if there is too much colour, you should disable it. (See section
4.3.4.)
In old versions of PuTTY, this was disabled by default, and would
not take effect until you reset the terminal (see question A.7.9).
Since 0.54, it is enabled by default, and changes take effect
immediately.
A.7.9 When I change some terminal settings, nothing happens.
Some of the terminal options (notably Auto Wrap and background-
colour screen erase) actually represent the _default_ setting,
rather than the currently active setting. The server can send
sequences that modify these options in mid-session, but when the
terminal is reset (by server action, or by you choosing `Reset
Terminal' from the System menu) the defaults are restored.
In versions 0.53b and prior, if you change one of these options in
the middle of a session, you will find that the change does not
immediately take effect. It will only take effect once you reset the
terminal.
In version 0.54, the behaviour has changed - changes to these
settings take effect immediately.
A.7.10 My PuTTY sessions unexpectedly close after they are idle for a
while.
Some types of firewall, and almost any router doing Network Address
Translation (NAT, also known as IP masquerading), will forget about
a connection through them if the connection does nothing for too
long. This will cause the connection to be rudely cut off when
contact is resumed.
You can try to combat this by telling PuTTY to send _keepalives_:
packets of data which have no effect on the actual session, but
which reassure the router or firewall that the network connection is
still active and worth remembering about.
Keepalives don't solve everything, unfortunately; although they
cause greater robustness against this sort of router, they can also
cause a _loss_ of robustness against network dropouts. See section
4.13.1 in the documentation for more discussion of this.
A.7.11 PuTTY's network connections time out too quickly when network
connectivity is temporarily lost.
This is a Windows problem, not a PuTTY problem. The timeout value
can't be set on per application or per session basis. To increase
the TCP timeout globally, you need to tinker with the Registry.
On Windows 95, 98 or ME, the registry key you need to create or
change is
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\
MSTCP\MaxDataRetries
(it must be of type DWORD in Win95, or String in Win98/ME). (See MS
Knowledge Base article 158474 for more information.)
On Windows NT, 2000, or XP, the registry key to create or change is
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\
Parameters\TcpMaxDataRetransmissions
and it must be of type DWORD. (See MS Knowledge Base articles 120642
and 314053 for more information.)
Set the key's value to something like 10. This will cause Windows to
try harder to keep connections alive instead of abandoning them.
A.7.12 When I cat a binary file, I get `PuTTYPuTTYPuTTY' on my command
line.
Don't do that, then.
This is designed behaviour; when PuTTY receives the character
Control-E from the remote server, it interprets it as a request
to identify itself, and so it sends back the string `PuTTY' as if
that string had been entered at the keyboard. Control-E should only
be sent by programs that are prepared to deal with the response.
Writing a binary file to your terminal is likely to output many
Control-E characters, and cause this behaviour. Don't do it. It's a
bad plan.
To mitigate the effects, you could configure the answerback string
to be empty (see section 4.3.6); but writing binary files to your
terminal is likely to cause various other unpleasant behaviour, so
this is only a small remedy.
A.7.13 When I cat a binary file, my window title changes to a nonsense
string.
Don't do that, then.
It is designed behaviour that PuTTY should have the ability
to adjust the window title on instructions from the server.
Normally the control sequence that does this should only be sent
deliberately, by programs that know what they are doing and intend
to put meaningful text in the window title. Writing a binary file to
your terminal runs the risk of sending the same control sequence by
accident, and cause unexpected changes in the window title. Don't do
it.
A.7.14 My keyboard stops working once PuTTY displays the password prompt.
No, it doesn't. PuTTY just doesn't display the password you type, so
that someone looking at your screen can't see what it is.
Unlike the Windows login prompts, PuTTY doesn't display the password
as a row of asterisks either. This is so that someone looking at
your screen can't even tell how _long_ your password is, which might
be valuable information.
A.7.15 One or more function keys don't do what I expected in a server-side
application.
If you've already tried all the relevant options in the PuTTY
Keyboard panel, you may need to mail the PuTTY maintainers and ask.
It is _not_ usually helpful just to tell us which application,
which server operating system, and which key isn't working; in
order to replicate the problem we would need to have a copy of
every operating system, and every application, that anyone has ever
complained about.
PuTTY responds to function key presses by sending a sequence of
control characters to the server. If a function key isn't doing what
you expect, it's likely that the character sequence your application
is expecting to receive is not the same as the one PuTTY is sending.
Therefore what we really need to know is _what_ sequence the
application is expecting.
The simplest way to investigate this is to find some other terminal
environment, in which that function key _does_ work; and then
investigate what sequence the function key is sending in that
situation. One reasonably easy way to do this on a Unix system is
to type the command `cat', and then press the function key. This is
likely to produce output of the form `^[[11~'. You can also do this
in PuTTY, to find out what sequence the function key is producing in
that. Then you can mail the PuTTY maintainers and tell us `I wanted
the F1 key to send `^[[11~', but instead it's sending `^[OP', can
this be done?', or something similar.
You should still read the Feedback page on the PuTTY website (also
provided as appendix B in the manual), and follow the guidelines
contained in that.
A.7.16 Since my SSH server was upgraded to OpenSSH 3.1p1/3.4p1, I can no
longer connect with PuTTY.
There is a known problem when OpenSSH has been built against an
incorrect version of OpenSSL; the quick workaround is to configure
PuTTY to use SSH protocol 2 and the Blowfish cipher.
For more details and OpenSSH patches, see bug 138 in the OpenSSH
BTS.
This is not a PuTTY-specific problem; if you try to connect with
another client you'll likely have similar problems. (Although
PuTTY's default cipher differs from many other clients.)
_OpenSSH 3.1p1:_ configurations known to be broken (and symptoms):
- SSH-2 with AES cipher (PuTTY says `Assertion failed! Expression:
(len & 15) == 0' in sshaes.c, or `Out of memory', or crashes)
- SSH-2 with 3DES (PuTTY says `Incorrect MAC received on packet')
- SSH-1 with Blowfish (PuTTY says `Incorrect CRC received on
packet')
- SSH-1 with 3DES
_OpenSSH 3.4p1:_ as of 3.4p1, only the problem with SSH-1 and
Blowfish remains. Rebuild your server, apply the patch linked to
from bug 138 above, or use another cipher (e.g., 3DES) instead.
_Other versions:_ we occasionally get reports of the same symptom
and workarounds with older versions of OpenSSH, although it's not
clear the underlying cause is the same.
A.7.17 Why do I see `Couldn't load private key from ...'? Why can PuTTYgen
load my key but not PuTTY?
It's likely that you've generated an SSH protocol 2 key with
PuTTYgen, but you're trying to use it in an SSH-1 connection. SSH-
1 and SSH-2 keys have different formats, and (at least in 0.52)
PuTTY's reporting of a key in the wrong format isn't optimal.
To connect using SSH-2 to a server that supports both versions, you
need to change the configuration from the default (see question
A.2.1).
A.7.18 When I'm connected to a Red Hat Linux 8.0 system, some characters
don't display properly.
A common complaint is that hyphens in man pages show up as a-acute.
With release 8.0, Red Hat appear to have made UTF-8 the default
character set. There appears to be no way for terminal emulators
such as PuTTY to know this (as far as we know, the appropriate
escape sequence to switch into UTF-8 mode isn't sent).
A fix is to configure sessions to RH8 systems to use UTF-8
translation - see section 4.10.1 in the documentation. (Note that if
you use `Change Settings', changes may not take place immediately -
see question A.7.9.)
If you really want to change the character set used by the server,
the right place is `/etc/sysconfig/i18n', but this shouldn't be
necessary.
A.7.19 Since I upgraded to PuTTY 0.54, the scrollback has stopped working
when I run `screen'.
PuTTY's terminal emulator has always had the policy that when the
`alternate screen' is in use, nothing is added to the scrollback.
This is because the usual sorts of programs which use the alternate
screen are things like text editors, which tend to scroll back and
forth in the same document a lot; so (a) they would fill up the
scrollback with a large amount of unhelpfully disordered text, and
(b) they contain their _own_ method for the user to scroll back to
the bit they were interested in. We have generally found this policy
to do the Right Thing in almost all situations.
Unfortunately, `screen' is one exception: it uses the alternate
screen, but it's still usually helpful to have PuTTY's scrollback
continue working. The simplest solution is to go to the Features
control panel and tick `Disable switching to alternate terminal
screen'. (See section 4.6.4 for more details.) Alternatively, you
can tell `screen' itself not to use the alternate screen: the
`screen' FAQ suggests adding the line `termcapinfo xterm ti@:te@' to
your .screenrc file.
The reason why this only started to be a problem in 0.54 is because
`screen' typically uses an unusual control sequence to switch to the
alternate screen, and previous versions of PuTTY did not support
this sequence.
A.7.20 Since I upgraded Windows XP to Service Pack 2, I can't use addresses
like 127.0.0.2.
Some people who ask PuTTY to listen on localhost addresses other
than 127.0.0.1 to forward services such as SMB and Windows Terminal
Services have found that doing so no longer works since they
upgraded to WinXP SP2.
This is apparently an issue with SP2 that is acknowledged by
Microsoft in MS Knowledge Base article 884020. The article links to
a fix you can download.
(_However_, we've been told that SP2 _also_ fixes the bug that means
you need to use non-127.0.0.1 addresses to forward Terminal Services
in the first place.)
A.7.21 PSFTP commands seem to be missing a directory separator (slash).
Some people have reported the following incorrect behaviour with
PSFTP:
psftp> pwd
Remote directory is /dir1/dir2
psftp> get filename.ext
/dir1/dir2filename.ext: no such file or directory
This is not a bug in PSFTP. There is a known bug in some versions of
portable OpenSSH (bug 697) that causes these symptoms; it appears
to have been introduced around 3.7.x. It manifests only on certain
platforms (AIX is what has been reported to us).
There is a patch for OpenSSH attached to that bug; it's also fixed
in recent versions of portable OpenSSH (from around 3.8).
A.7.22 Do you want to hear about `Software caused connection abort'?
In the documentation for PuTTY 0.53 and 0.53b, we mentioned that
we'd like to hear about any occurrences of this error. Since the
release of PuTTY 0.54, however, we've been convinced that this error
doesn't indicate that PuTTY's doing anything wrong, and we don't
need to hear about further occurrences. See section 10.14 for our
current documentation of this error.
A.7.23 My SSH-2 session locks up for a few seconds every so often.
Recent versions of PuTTY automatically initiate repeat key exchange
once per hour, to improve session security. If your client or server
machine is slow, you may experience this as a delay of anything up
to thirty seconds or so.
These delays are inconvenient, but they are there for your
protection. If they really cause you a problem, you can choose to
turn off periodic rekeying using the `Kex' configuration panel (see
section 4.19), but be aware that you will be sacrificing security
for this. (Falling back to SSH-1 would also remove the delays, but
would lose a _lot_ more security still. We do not recommend it.)
A.8 Security questions
A.8.1 Is it safe for me to download PuTTY and use it on a public PC?
It depends on whether you trust that PC. If you don't trust the
public PC, don't use PuTTY on it, and don't use any other software
you plan to type passwords into either. It might be watching your
keystrokes, or it might tamper with the PuTTY binary you download.
There is _no_ program safe enough that you can run it on an actively
malicious PC and get away with typing passwords into it.
If you do trust the PC, then it's probably OK to use PuTTY on it
(but if you don't trust the network, then the PuTTY download might
be tampered with, so it would be better to carry PuTTY with you on a
floppy).
A.8.2 What does PuTTY leave on a system? How can I clean up after it?
PuTTY will leave some Registry entries, and a random seed file, on
the PC (see question A.5.2). If you are using PuTTY on a public
PC, or somebody else's PC, you might want to clean these up when
you leave. You can do that automatically, by running the command
`putty -cleanup'. (Note that this only removes settings for the
currently logged-in user on multi-user systems.)
If PuTTY was installed from the installer package, it will also
appear in `Add/Remove Programs'. Older versions of the uninstaller
do not remove the above-mentioned registry entries and file.
A.8.3 How come PuTTY now supports DSA, when the website used to say how
insecure it was?
DSA has a major weakness _if badly implemented_: it relies on a
random number generator to far too great an extent. If the random
number generator produces a number an attacker can predict, the DSA
private key is exposed - meaning that the attacker can log in as you
on all systems that accept that key.
The PuTTY policy changed because the developers were informed of
ways to implement DSA which do not suffer nearly as badly from this
weakness, and indeed which don't need to rely on random numbers at
all. For this reason we now believe PuTTY's DSA implementation is
probably OK. However, if you have the choice, we still recommend you
use RSA instead.
A.8.4 Couldn't Pageant use VirtualLock() to stop private keys being
written to disk?
Unfortunately not. The VirtualLock() function in the Windows API
doesn't do a proper job: it may prevent small pieces of a process's
memory from being paged to disk while the process is running, but
it doesn't stop the process's memory as a whole from being swapped
completely out to disk when the process is long-term inactive. And
Pageant spends most of its time inactive.
A.9 Administrative questions
A.9.1 Would you like me to register you a nicer domain name?
No, thank you. Even if you can find one (most of them seem to
have been registered already, by people who didn't ask whether we
actually wanted it before they applied), we're happy with the PuTTY
web site being exactly where it is. It's not hard to find (just type
`putty' into google.com and we're the first link returned), and we
don't believe the administrative hassle of moving the site would be
worth the benefit.
In addition, if we _did_ want a custom domain name, we would want to
run it ourselves, so we knew for certain that it would continue to
point where we wanted it, and wouldn't suddenly change or do strange
things. Having it registered for us by a third party who we don't
even know is not the best way to achieve this.
A.9.2 Would you like free web hosting for the PuTTY web site?
We already have some, thanks.
A.9.3 Would you link to my web site from the PuTTY web site?
Only if the content of your web page is of definite direct interest
to PuTTY users. If your content is unrelated, or only tangentially
related, to PuTTY, then the link would simply be advertising for
you.
One very nice effect of the Google ranking mechanism is that by and
large, the most popular web sites get the highest rankings. This
means that when an ordinary person does a search, the top item in
the search is very likely to be a high-quality site or the site they
actually wanted, rather than the site which paid the most money for
its ranking.
The PuTTY web site is held in high esteem by Google, for precisely
this reason: lots of people have linked to it simply because they
like PuTTY, without us ever having to ask anyone to link to us. We
feel that it would be an abuse of this esteem to use it to boost the
ranking of random advertisers' web sites. If you want your web site
to have a high Google ranking, we'd prefer that you achieve this the
way we did - by being good enough at what you do that people will
link to you simply because they like you.
In particular, we aren't interested in trading links for money
(see above), and we _certainly_ aren't interested in trading links
for other links (since we have no advertising on our web site, our
Google ranking is not even directly worth anything to us). If we
don't want to link to you for free, then we probably won't want to
link to you at all.
If you have software based on PuTTY, or specifically designed to
interoperate with PuTTY, or in some other way of genuine interest
to PuTTY users, then we will probably be happy to add a link to you
on our Links page. And if you're running a mirror of the PuTTY web
site, we're _definitely_ interested.
A.9.4 Why don't you move PuTTY to SourceForge?
Partly, because we don't want to move the web site location (see
question A.9.1).
Also, security reasons. PuTTY is a security product, and as such it
is particularly important to guard the code and the web site against
unauthorised modifications which might introduce subtle security
flaws. Therefore, we prefer that the Subversion repository, web
site and FTP site remain where they are, under the direct control
of system administrators we know and trust personally, rather than
being run by a large organisation full of people we've never met and
which is known to have had breakins in the past.
No offence to SourceForge; I think they do a wonderful job. But
they're not ideal for everyone, and in particular they're not ideal
for us.
A.9.5 Why can't I subscribe to the putty-bugs mailing list?
Because you're not a member of the PuTTY core development team. The
putty-bugs mailing list is not a general newsgroup-like discussion
forum; it's a contact address for the core developers, and an
_internal_ mailing list for us to discuss things among ourselves.
If we opened it up for everybody to subscribe to, it would turn
into something more like a newsgroup and we would be completely
overwhelmed by the volume of traffic. It's hard enough to keep up
with the list as it is.
A.9.6 If putty-bugs isn't a general-subscription mailing list, what is?
There isn't one, that we know of.
If someone else wants to set up a mailing list or other forum for
PuTTY users to help each other with common problems, that would
be fine with us, though the PuTTY team would almost certainly not
have the time to read it. It's probably better to use one of the
established newsgroups for this purpose (see section B.1.2).
A.9.7 How can I donate to PuTTY development?
Please, _please_ don't feel you have to. PuTTY is completely free
software, and not shareware. We think it's very important that
_everybody_ who wants to use PuTTY should be able to, whether they
have any money or not; so the last thing we would want is for a
PuTTY user to feel guilty because they haven't paid us any money. If
you want to keep your money, please do keep it. We wouldn't dream of
asking for any.
Having said all that, if you still really _want_ to give us money,
we won't argue :-) The easiest way for us to accept donations is if
you send money to using PayPal (www.paypal.com).
Alternatively, if you don't trust PayPal, you could donate through
e-gold (www.e-gold.com): deposit your donation in account number
174769, then send us e-mail to let us know you've done so (otherwise
we might not notice for months!).
Small donations (tens of dollars or tens of euros) will probably
be spent on beer or curry, which helps motivate our volunteer team
to continue doing this for the world. Larger donations will be
spent on something that actually helps development, if we can find
anything (perhaps new hardware, or a copy of Windows XP), but if we
can't find anything then we'll just distribute the money among the
developers. If you want to be sure your donation is going towards
something worthwhile, ask us first. If you don't like these terms,
feel perfectly free not to donate. We don't mind.
A.9.8 Can I have permission to put PuTTY on a cover disk / distribute it
with other software / etc?
Yes. You need not bother asking us explicitly for permission. You
already have permission. Redistribution of the unmodified PuTTY
binary in this way is entirely permitted by our licence (see
appendix C), and you are welcome to do it as much as you like.
If you are distributing PuTTY within your own organisation, or for
use with your own product, then we recommend (but do not insist)
that you offer your own first-line technical support, to answer
questions directly relating to the interaction of PuTTY with your
particular environment. If your users mail us directly, we won't be
able to give them very much help about things specific to your own
setup.
A.10 Miscellaneous questions
A.10.1 Is PuTTY a port of OpenSSH, or based on OpenSSH?
No, it isn't. PuTTY is almost completely composed of code written
from scratch for PuTTY. The only code we share with OpenSSH is the
detector for SSH-1 CRC compensation attacks, written by CORE SDI
S.A.
A.10.2 Where can I buy silly putty?
You're looking at the wrong web site; the only PuTTY we know about
here is the name of a computer program.
If you want the kind of putty you can buy as an executive toy, the
PuTTY team can personally recommend Thinking Putty, which you can
buy from Crazy Aaron's Putty World, at www.puttyworld.com.
A.10.3 What does `PuTTY' mean?
It's the name of a popular SSH and Telnet client. Any other meaning
is in the eye of the beholder. It's been rumoured that `PuTTY' is
the antonym of `getty', or that it's the stuff that makes your
Windows useful, or that it's a kind of plutonium Teletype. We
couldn't possibly comment on such allegations.
A.10.4 How do I pronounce `PuTTY'?
Exactly like the English word `putty', which we pronounce /'pVtI/.
Appendix B: Feedback and bug reporting
--------------------------------------
This is a guide to providing feedback to the PuTTY development team.
It is provided as both a web page on the PuTTY site, and an appendix
in the PuTTY manual.
Section B.1 gives some general guidelines for sending any kind of e-
mail to the development team. Following sections give more specific
guidelines for particular types of e-mail, such as bug reports and
feature requests.
B.1 General guidelines
The PuTTY development team gets a _lot_ of mail. If you can possibly
solve your own problem by reading the manual, reading the FAQ,
reading the web site, asking a fellow user, perhaps posting to a
newsgroup (see section B.1.2), or some other means, then it would
make our lives much easier.
We get so much e-mail that we literally do not have time to answer
it all. We regret this, but there's nothing we can do about it.
So if you can _possibly_ avoid sending mail to the PuTTY team, we
recommend you do so. In particular, support requests (section B.5)
are probably better sent to newsgroups, or passed to a local expert
if possible.
The PuTTY contact email address is a private mailing list containing
four or five core developers. Don't be put off by it being a mailing
list: if you need to send confidential data as part of a bug report,
you can trust the people on the list to respect that confidence.
Also, the archives aren't publicly available, so you shouldn't be
letting yourself in for any spam by sending us mail.
Please use a meaningful subject line on your message. We get a lot
of mail, and it's hard to find the message we're looking for if they
all have subject lines like `PuTTY bug'.
B.1.1 Sending large attachments
Since the PuTTY contact address is a mailing list, e-mails larger
than 40Kb will be held for inspection by the list administrator, and
will not be allowed through unless they really appear to be worth
their large size.
If you are considering sending any kind of large data file to the
PuTTY team, it's almost always a bad idea, or at the very least it
would be better to ask us first whether we actually need the file.
Alternatively, you could put the file on a web site and just send us
the URL; that way, we don't have to download it unless we decide we
actually need it, and only one of us needs to download it instead of
it being automatically copied to all the developers.
Some people like to send mail in MS Word format. Please _don't_
send us bug reports, or any other mail, as a Word document. Word
documents are roughly fifty times larger than writing the same
report in plain text. In addition, most of the PuTTY team read their
e-mail on Unix machines, so copying the file to a Windows box to run
Word is very inconvenient. Not only that, but several of us don't
even _have_ a copy of Word!
Some people like to send us screen shots when demonstrating a
problem. Please don't do this without checking with us first
- we almost never actually need the information in the screen
shot. Sending a screen shot of an error box is almost certainly
unnecessary when you could just tell us in plain text what the
error was. (On some versions of Windows, pressing Ctrl-C when the
error box is displayed will copy the text of the message to the
clipboard.) Sending a full-screen shot is _occasionally_ useful, but
it's probably still wise to check whether we need it before sending
it.
If you _must_ mail a screen shot, don't send it as a .BMP file. BMPs
have no compression and they are _much_ larger than other image
formats such as PNG, TIFF and GIF. Convert the file to a properly
compressed image format before sending it.
Please don't mail us executables, at all. Our mail server blocks all
incoming e-mail containing executables, as a defence against the
vast numbers of e-mail viruses we receive every day. If you mail us
an executable, it will just bounce.
If you have made a tiny modification to the PuTTY code, please
send us a _patch_ to the source code if possible, rather than
sending us a huge .ZIP file containing the complete sources plus
your modification. If you've only changed 10 lines, we'd prefer to
receive a mail that's 30 lines long than one containing multiple
megabytes of data we already have.
B.1.2 Other places to ask for help
There are two Usenet newsgroups that are particularly relevant to
the PuTTY tools:
- `comp.security.ssh', for questions specific to using the SSH
protocol;
- `comp.terminals', for issues relating to terminal emulation (for
instance, keyboard problems).
If you don't have direct access to Usenet, you can access these
newsgroups through Google Groups (groups.google.com).
B.2 Reporting bugs
If you think you have found a bug in PuTTY, your first steps should
be:
- Check the Wishlist page on the PuTTY website, and see if we
already know about the problem. If we do, it is almost certainly
not necessary to mail us about it, unless you think you have
extra information that might be helpful to us in fixing it. (Of
course, if we actually _need_ specific extra information about a
particular bug, the Wishlist page will say so.)
- Check the Change Log on the PuTTY website, and see if we have
already fixed the bug in the development snapshots.
- Check the FAQ on the PuTTY website (also provided as appendix
A in the manual), and see if it answers your question. The FAQ
lists the most common things which people think are bugs, but
which aren't bugs.
- Download the latest development snapshot and see if the problem
still happens with that. This really is worth doing. As a
general rule we aren't very interested in bugs that appear in
the release version but not in the development version, because
that usually means they are bugs we have _already fixed_. On the
other hand, if you can find a bug in the development version
that doesn't appear in the release, that's likely to be a new
bug we've introduced since the release and we're definitely
interested in it.
If none of those options solved your problem, and you still need
to report a bug to us, it is useful if you include some general
information:
- Tell us what version of PuTTY you are running. To find this
out, use the `About PuTTY' option from the System menu. Please
_do not_ just tell us `I'm running the latest version'; e-mail
can be delayed and it may not be obvious which version was the
latest at the time you sent the message.
- PuTTY is a multi-platform application; tell us what version of
what OS you are running PuTTY on. (If you're running on Unix, or
Windows for Alpha, tell us, or we'll assume you're running on
Windows for Intel as this is overwhelmingly the case.)
- Tell us what protocol you are connecting with: SSH, Telnet,
Rlogin or Raw mode.
- Tell us what kind of server you are connecting to; what OS, and
if possible what SSH server (if you're using SSH). You can get
some of this information from the PuTTY Event Log (see section
3.1.3.1 in the manual).
- Send us the contents of the PuTTY Event Log, unless you have a
specific reason not to (for example, if it contains confidential
information that you think we should be able to solve your
problem without needing to know).
- Try to give us as much information as you can to help us see
the problem for ourselves. If possible, give us a step-by-step
sequence of _precise_ instructions for reproducing the fault.
- Don't just tell us that PuTTY `does the wrong thing'; tell us
exactly and precisely what it did, and also tell us exactly
and precisely what you think it should have done instead. Some
people tell us PuTTY does the wrong thing, and it turns out that
it was doing the right thing and their expectations were wrong.
Help to avoid this problem by telling us exactly what you think
it should have done, and exactly what it did do.
- If you think you can, you're welcome to try to fix the
problem yourself. A patch to the code which fixes a bug is an
excellent addition to a bug report. However, a patch is never a
_substitute_ for a good bug report; if your patch is wrong or
inappropriate, and you haven't supplied us with full information
about the actual bug, then we won't be able to find a better
solution.
- http://www.chiark.greenend.org.uk/~sgtatham/bugs.html is an
article on how to report bugs effectively in general. If your
bug report is _particularly_ unclear, we may ask you to go away,
read this article, and then report the bug again.
It is reasonable to report bugs in PuTTY's documentation, if you
think the documentation is unclear or unhelpful. But we do need
to be given exact details of _what_ you think the documentation
has failed to tell you, or _how_ you think it could be made
clearer. If your problem is simply that you don't _understand_ the
documentation, we suggest posting to a newsgroup (see section B.1.2)
and seeing if someone will explain what you need to know. _Then_, if
you think the documentation could usefully have told you that, send
us a bug report and explain how you think we should change it.
B.3 Requesting extra features
If you want to request a new feature in PuTTY, the very first things
you should do are:
- Check the Wishlist page on the PuTTY website, and see if your
feature is already on the list. If it is, it probably won't
achieve very much to repeat the request. (But see section B.4 if
you want to persuade us to give your particular feature higher
priority.)
- Check the Wishlist and Change Log on the PuTTY website, and
see if we have already added your feature in the development
snapshots. If it isn't clear, download the latest development
snapshot and see if the feature is present. If it is, then it
will also be in the next release and there is no need to mail us
at all.
If you can't find your feature in either the development snapshots
_or_ the Wishlist, then you probably do need to submit a feature
request. Since the PuTTY authors are very busy, it helps if you try
to do some of the work for us:
- Do as much of the design as you can. Think about `corner cases';
think about how your feature interacts with other existing
features. Think about the user interface; if you can't come
up with a simple and intuitive interface to your feature, you
shouldn't be surprised if we can't either. Always imagine
whether it's possible for there to be more than one, or less
than one, of something you'd assumed there would be one of.
(For example, if you were to want PuTTY to put an icon in the
System tray rather than the Taskbar, you should think about what
happens if there's more than one PuTTY active; how would the
user tell which was which?)
- If you can program, it may be worth offering to write the
feature yourself and send us a patch. However, it is likely to
be helpful if you confer with us first; there may be design
issues you haven't thought of, or we may be about to make big
changes to the code which your patch would clash with, or
something. If you check with the maintainers first, there is a
better chance of your code actually being usable. Also, read the
design principles listed in appendix D: if you do not conform to
them, we will probably not be able to accept your patch.
B.4 Requesting features that have already been requested
If a feature is already listed on the Wishlist, then it usually
means we would like to add it to PuTTY at some point. However, this
may not be in the near future. If there's a feature on the Wishlist
which you would like to see in the _near_ future, there are several
things you can do to try to increase its priority level:
- Mail us and vote for it. (Be sure to mention that you've seen
it on the Wishlist, or we might think you haven't even _read_
the Wishlist). This probably won't have very _much_ effect; if
a huge number of people vote for something then it may make a
difference, but one or two extra votes for a particular feature
are unlikely to change our priority list immediately. Offering a
new and compelling justification might help. Also, don't expect
a reply.
- Offer us money if we do the work sooner rather than later. This
sometimes works, but not always. The PuTTY team all have full-
time jobs and we're doing all of this work in our free time; we
may sometimes be willing to give up some more of our free time
in exchange for some money, but if you try to bribe us for a
_big_ feature it's entirely possible that we simply won't have
the time to spare - whether you pay us or not. (Also, we don't
accept bribes to add _bad_ features to the Wishlist, because
our desire to provide high-quality software to the users comes
first.)
- Offer to help us write the code. This is probably the _only_ way
to get a feature implemented quickly, if it's a big one that we
don't have time to do ourselves.
B.5 Support requests
If you're trying to make PuTTY do something for you and it isn't
working, but you're not sure whether it's a bug or not, then
_please_ consider looking for help somewhere else. This is one of
the most common types of mail the PuTTY team receives, and we simply
don't have time to answer all the questions. Questions of this type
include:
- If you want to do something with PuTTY but have no idea where to
start, and reading the manual hasn't helped, try posting to a
newsgroup (see section B.1.2) and see if someone can explain it
to you.
- If you have tried to do something with PuTTY but it hasn't
worked, and you aren't sure whether it's a bug in PuTTY or a bug
in your SSH server or simply that you're not doing it right,
then try posting to a newsgroup (see section B.1.2) and see if
someone can solve your problem. Or try doing the same thing with
a different SSH client and see if it works with that. Please do
not report it as a PuTTY bug unless you are really sure it _is_
a bug in PuTTY.
- If someone else installed PuTTY for you, or you're using PuTTY
on someone else's computer, try asking them for help first.
They're more likely to understand how they installed it and what
they expected you to use it for than we are.
- If you have successfully made a connection to your server and
now need to know what to type at the server's command prompt,
or other details of how to use the server-end software, talk to
your server's system administrator. This is not the PuTTY team's
problem. PuTTY is only a communications tool, like a telephone;
if you can't speak the same language as the person at the other
end of the phone, it isn't the telephone company's job to teach
it to you.
If you absolutely cannot get a support question answered any other
way, you can try mailing it to us, but we can't guarantee to have
time to answer it.
B.6 Web server administration
If the PuTTY web site is down (Connection Timed Out), please don't
bother mailing us to tell us about it. Most of us read our e-mail on
the same machines that host the web site, so if those machines are
down then we will notice _before_ we read our e-mail. So there's no
point telling us our servers are down.
Of course, if the web site has some other error (Connection Refused,
404 Not Found, 403 Forbidden, or something else) then we might _not_
have noticed and it might still be worth telling us about it.
If you want to report a problem with our web site, check that you're
looking at our _real_ web site and not a mirror. The real web site
is at `http://www.chiark.greenend.org.uk/~sgtatham/putty/'; if
that's not where you're reading this, then don't report the problem
to us until you've checked that it's really a problem with the
main site. If it's only a problem with the mirror, you should try
to contact the administrator of that mirror site first, and only
contact us if that doesn't solve the problem (in case we need to
remove the mirror from our list).
B.7 Asking permission for things
PuTTY is distributed under the MIT Licence (see appendix C for
details). This means you can do almost _anything_ you like with our
software, our source code, and our documentation. The only things
you aren't allowed to do are to remove our copyright notices or the
licence text itself, or to hold us legally responsible if something
goes wrong.
So if you want permission to include PuTTY on a magazine cover disk,
or as part of a collection of useful software on a CD or a web site,
then _permission is already granted_. You don't have to mail us and
ask. Just go ahead and do it. We don't mind.
(If you want to distribute PuTTY alongside your own application for
use with that application, or if you want to distribute PuTTY within
your own organisation, then we recommend you offer your own first-
line technical support, to answer questions about the interaction
of PuTTY with your environment. If your users mail us directly,
we won't be able to tell them anything useful about your specific
setup.)
If you want to use parts of the PuTTY source code in another
program, then it might be worth mailing us to talk about technical
details, but if all you want is to ask permission then you don't
need to bother. You already have permission.
B.8 Mirroring the PuTTY web site
Mirrors of the PuTTY web site are welcome, especially in regions not
well covered by existing mirrors. Please don't bother asking us for
permission before setting up a mirror. You already have permission.
We are always happy to have more mirrors.
If you mail us _after_ you have set up the mirror and checked that
it works, and remember to let us know which country your mirror is
in, then we'll add it to the Mirrors page on the PuTTY website.
If you have technical questions about the process of mirroring, then
you might want to mail us before setting up the mirror (see also the
guidelines on the Mirrors page); but if you just want to ask for
permission, you don't need to. You already have permission.
B.9 Praise and compliments
One of the most rewarding things about maintaining free software
is getting e-mails that just say `thanks'. We are always happy to
receive e-mails of this type.
Regrettably we don't have time to answer them all in person. If you
mail us a compliment and don't receive a reply, _please_ don't think
we've ignored you. We did receive it and we were happy about it; we
just didn't have time to tell you so personally.
To everyone who's ever sent us praise and compliments, in the past
and the future: _you're welcome_!
B.10 E-mail address
The actual address to mail is .
Appendix C: PuTTY Licence
-------------------------
PuTTY is copyright 1997-2005 Simon Tatham.
Portions copyright Robert de Bath, Joris van Rantwijk, Delian
Delchev, Andreas Schultz, Jeroen Massar, Wez Furlong, Nicolas Barry,
Justin Bradford, Ben Harris, Malcolm Smith, Markus Kuhn, and CORE
SDI S.A.
Permission is hereby granted, free of charge, to any person
obtaining a copy of this software and associated documentation files
(the `Software'), to deal in the Software without restriction,
including without limitation the rights to use, copy, modify, merge,
publish, distribute, sublicense, and/or sell copies of the Software,
and to permit persons to whom the Software is furnished to do so,
subject to the following conditions:
The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED `AS IS', WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE
FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF
CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
Appendix D: PuTTY hacking guide
-------------------------------
This appendix lists a selection of the design principles applying
to the PuTTY source code. If you are planning to send code
contributions, you should read this first.
D.1 Cross-OS portability
Despite Windows being its main area of fame, PuTTY is no longer a
Windows-only application suite. It has a working Unix port; a Mac
port is in progress; more ports may or may not happen at a later
date.
Therefore, embedding Windows-specific code in core modules such as
ssh.c is not acceptable. We went to great lengths to _remove_ all
the Windows-specific stuff from our core modules, and to shift it
out into Windows-specific modules. Adding large amounts of Windows-
specific stuff in parts of the code that should be portable is
almost guaranteed to make us reject a contribution.
The PuTTY source base is divided into platform-specific modules
and platform-generic modules. The Unix-specific modules are all in
the `unix' subdirectory; the Mac-specific modules are in the `mac'
subdirectory; the Windows-specific modules are in the `windows'
subdirectory.
All the modules in the main source directory - notably _all_ of the
code for the various back ends - are platform-generic. We want to
keep them that way.
This also means you should stick to what you are guaranteed by
ANSI/ISO C (that is, the original C89/C90 standard, not C99). Try
not to make assumptions about the precise size of basic types such
as `int' and `long int'; don't use pointer casts to do endianness-
dependent operations, and so on.
(There are one or two aspects of ANSI C portability which we _don't_
care about. In particular, we expect PuTTY to be compiled on 32-
bit architectures _or bigger_; so it's safe to assume that `int' is
at least 32 bits wide, not just the 16 you are guaranteed by ANSI
C. Similarly, we assume that the execution character encoding is
a superset of the printable characters of ASCII, though we don't
assume the numeric values of control characters, particularly '\n'
and '\r'.)
D.2 Multiple backends treated equally
PuTTY is not an SSH client with some other stuff tacked on the side.
PuTTY is a generic, multiple-backend, remote VT-terminal client
which happens to support one backend which is larger, more popular
and more useful than the rest. Any extra feature which can possibly
be general across all backends should be so: localising features
unnecessarily into the SSH back end is a design error. (For example,
we had several code submissions for proxy support which worked by
hacking ssh.c. Clearly this is completely wrong: the network.h
abstraction is the place to put it, so that it will apply to all
back ends equally, and indeed we eventually put it there after
another contributor sent a better patch.)
The rest of PuTTY should try to avoid knowing anything about
specific back ends if at all possible. To support a feature which is
only available in one network protocol, for example, the back end
interface should be extended in a general manner such that _any_
back end which is able to provide that feature can do so. If it so
happens that only one back end actually does, that's just the way it
is, but it shouldn't be relied upon by any code.
D.3 Multiple sessions per process on some platforms
Some ports of PuTTY - notably the in-progress Mac port - are
constrained by the operating system to run as a single process
potentially managing multiple sessions.
Therefore, the platform-independent parts of PuTTY never use global
variables to store per-session data. The global variables that do
exist are tolerated because they are not specific to a particular
login session: `flags' defines properties that are expected to apply
equally to _all_ the sessions run by a single PuTTY process, the
random number state in sshrand.c and the timer list in timing.c
serve all sessions equally, and so on. But most data is specific to
a particular network session, and is therefore stored in dynamically
allocated data structures, and pointers to these structures are
passed around between functions.
Platform-specific code can reverse this decision if it likes. The
Windows code, for historical reasons, stores most of its data as
global variables. That's OK, because _on Windows_ we know there is
only one session per PuTTY process, so it's safe to do that. But
changes to the platform-independent code should avoid introducing
global variables, unless they are genuinely cross-session.
D.4 C, not C++
PuTTY is written entirely in C, not in C++.
We have made _some_ effort to make it easy to compile our code using
a C++ compiler: notably, our `snew', `snewn' and `sresize' macros
explicitly cast the return values of malloc and realloc to the
target type. (This has type checking advantages even in C: it means
you never accidentally allocate the wrong size piece of memory for
the pointer type you're assigning it to. C++ friendliness is really
a side benefit.)
We want PuTTY to continue being pure C, at least in the platform-
independent parts and the currently existing ports. Patches which
switch the Makefiles to compile it as C++ and start using classes
will not be accepted. Also, in particular, we disapprove of //
comments, at least for the moment. (Perhaps once C99 becomes
genuinely widespread we might be more lenient.)
The one exception: a port to a new platform may use languages other
than C if they are necessary to code on that platform. If your
favourite PDA has a GUI with a C++ API, then there's no way you can
do a port of PuTTY without using C++, so go ahead and use it. But
keep the C++ restricted to that platform's subdirectory; if your
changes force the Unix or Windows ports to be compiled as C++, they
will be unacceptable to us.
D.5 Security-conscious coding
PuTTY is a network application and a security application. Assume
your code will end up being fed deliberately malicious data by
attackers, and try to code in a way that makes it unlikely to be a
security risk.
In particular, try not to use fixed-size buffers for variable-size
data such as strings received from the network (or even the user).
We provide functions such as dupcat and dupprintf, which dynamically
allocate buffers of the right size for the string they construct.
Use these wherever possible.
D.6 Independence of specific compiler
Windows PuTTY can currently be compiled with any of four Windows
compilers: MS Visual C, Borland's freely downloadable C compiler,
the Cygwin / mingw32 GNU tools, and lcc-win32.
This is a really useful property of PuTTY, because it means people
who want to contribute to the coding don't depend on having a
specific compiler; so they don't have to fork out money for MSVC if
they don't already have it, but on the other hand if they _do_ have
it they also don't have to spend effort installing gcc alongside it.
They can use whichever compiler they happen to have available, or
install whichever is cheapest and easiest if they don't have one.
Therefore, we don't want PuTTY to start depending on which compiler
you're using. Using GNU extensions to the C language, for example,
would ruin this useful property (not that anyone's ever tried
it!); and more realistically, depending on an MS-specific library
function supplied by the MSVC C library (_snprintf, for example)
is a mistake, because that function won't be available under the
other compilers. Any function supplied in an official Windows DLL
as part of the Windows API is fine, and anything defined in the C
library standard is also fine, because those should be available
irrespective of compilation environment. But things in between,
available as non-standard library and language extensions in only
one compiler, are disallowed.
(_snprintf in particular should be unnecessary, since we provide
dupprintf; see section D.5.)
Compiler independence should apply on all platforms, of course, not
just on Windows.
D.7 Small code size
PuTTY is tiny, compared to many other Windows applications. And
it's easy to install: it depends on no DLLs, no other applications,
no service packs or system upgrades. It's just one executable. You
install that executable wherever you want to, and run it.
We want to keep both these properties - the small size, and the ease
of installation - if at all possible. So code contributions that
depend critically on external DLLs, or that add a huge amount to the
code size for a feature which is only useful to a small minority of
users, are likely to be thrown out immediately.
We do vaguely intend to introduce a DLL plugin interface for PuTTY,
whereby seriously large extra features can be implemented in plugin
modules. The important thing, though, is that those DLLs will be
_optional_; if PuTTY can't find them on startup, it should run
perfectly happily and just won't provide those particular features.
A full installation of PuTTY might one day contain ten or twenty
little DLL plugins, which would cut down a little on the ease of
installation - but if you really needed ease of installation you
_could_ still just install the one PuTTY binary, or just the DLLs
you really needed, and it would still work fine.
Depending on _external_ DLLs is something we'd like to avoid if
at all possible (though for some purposes, such as complex SSH
authentication mechanisms, it may be unavoidable). If it can't be
avoided, the important thing is to follow the same principle of
graceful degradation: if a DLL can't be found, then PuTTY should run
happily and just not supply the feature that depended on it.
D.8 Single-threaded code
PuTTY and its supporting tools, or at least the vast majority of
them, run in only one OS thread.
This means that if you're devising some piece of internal mechanism,
there's no need to use locks to make sure it doesn't get called by
two threads at once. The only way code can be called re-entrantly is
by recursion.
That said, most of Windows PuTTY's network handling is triggered
off Windows messages requested by WSAAsyncSelect(), so if you call
MessageBox() deep within some network event handling code you should
be aware that you might be re-entered if a network event comes in
and is passed on to our window procedure by the MessageBox() message
loop.
Also, the front ends (in particular Windows Plink) can use multiple
threads if they like. However, Windows Plink keeps _very_ tight
control of its auxiliary threads, and uses them pretty much
exclusively as a form of select(). Pretty much all the code outside
windows/winplink.c is _only_ ever called from the one primary
thread; the others just loop round blocking on file handles and
send messages to the main thread when some real work needs doing.
This is not considered a portability hazard because that bit of
windows/winplink.c will need rewriting on other platforms in any
case.
One important consequence of this: PuTTY has only one thread in
which to do everything. That `everything' may include managing
more than one login session (section D.3), managing multiple data
channels within an SSH session, responding to GUI events even when
nothing is happening on the network, and responding to network
requests from the server (such as repeat key exchange) even when the
program is dealing with complex user interaction such as the re-
configuration dialog box. This means that _almost none_ of the PuTTY
code can safely block.
D.9 Keystrokes sent to the server wherever possible
In almost all cases, PuTTY sends keystrokes to the server. Even
weird keystrokes that you think should be hot keys controlling
PuTTY. Even Alt-F4 or Alt-Space, for example. If a keystroke has a
well-defined escape sequence that it could usefully be sending to
the server, then it should do so, or at the very least it should be
configurably able to do so.
To unconditionally turn a key combination into a hot key to control
PuTTY is almost always a design error. If a hot key is really truly
required, then try to find a key combination for it which _isn't_
already used in existing PuTTYs (either it sends nothing to the
server, or it sends the same thing as some other combination).
Even then, be prepared for the possibility that one day that key
combination might end up being needed to send something to the
server - so make sure that there's an alternative way to invoke
whatever PuTTY feature it controls.
D.10 640x480 friendliness in configuration panels
There's a reason we have lots of tiny configuration panels instead
of a few huge ones, and that reason is that not everyone has a
1600x1200 desktop. 640x480 is still a viable resolution for running
Windows (and indeed it's still the default if you start up in safe
mode), so it's still a resolution we care about.
Accordingly, the PuTTY configuration box, and the PuTTYgen control
window, are deliberately kept just small enough to fit comfortably
on a 640x480 display. If you're adding controls to either of these
boxes and you find yourself wanting to increase the size of the
whole box, _don't_. Split it into more panels instead.
D.11 Automatically generated Makefiles
PuTTY is intended to compile on multiple platforms, and with
multiple compilers. It would be horrifying to try to maintain a
single Makefile which handled all possible situations, and just as
painful to try to directly maintain a set of matching Makefiles for
each different compilation environment.
Therefore, we have moved the problem up by one level. In the PuTTY
source archive is a file called `Recipe', which lists which source
files combine to produce which binaries; and there is also a script
called mkfiles.pl, which reads `Recipe' and writes out the real
Makefiles. (The script also reads all the source files and analyses
their dependencies on header files, so we get an extra benefit from
doing it this way, which is that we can supply correct dependency
information even in environments where it's difficult to set up an
automated `make depend' phase.)
You should _never_ edit any of the PuTTY Makefiles directly.
They are not stored in our source repository at all. They are
automatically generated by mkfiles.pl from the file `Recipe'.
If you need to add a new object file to a particular binary, the
right thing to do is to edit `Recipe' and re-run mkfiles.pl. This
will cause the new object file to be added in every tool that
requires it, on every platform where it matters, in every Makefile
to which it is relevant, _and_ to get all the dependency data right.
If you send us a patch that modifies one of the Makefiles, you just
waste our time, because we will have to convert it into a change
to `Recipe'. If you send us a patch that modifies _all_ of the
Makefiles, you will have wasted a lot of _your_ time as well!
(There is a comment at the top of every Makefile in the PuTTY source
archive saying this, but many people don't seem to read it, so it's
worth repeating here.)
D.12 Coroutines in ssh.c
Large parts of the code in ssh.c are structured using a set
of macros that implement (something close to) Donald Knuth's
`coroutines' concept in C.
Essentially, the purpose of these macros are to arrange that a
function can call crReturn() to return to its caller, and the next
time it is called control will resume from just after that crReturn
statement.
This means that any local (automatic) variables declared in such
a function will be corrupted every time you call crReturn. If you
need a variable to persist for longer than that, you _must_ make it
a field in one of the persistent state structures: either the local
state structures `s' or `st' in each function, or the backend-wide
structure `ssh'.
See `http://www.chiark.greenend.org.uk/~sgtatham/coroutines.html'
for a more in-depth discussion of what these macros are for and how
they work.
D.13 Single compilation of each source file
The PuTTY build system for any given platform works on the following
very simple model:
- Each source file is compiled precisely once, to produce a single
object file.
- Each binary is created by linking together some combination of
those object files.
Therefore, if you need to introduce functionality to a particular
module which is only available in some of the tool binaries (for
example, a cryptographic proxy authentication mechanism which needs
to be left out of PuTTYtel to maintain its usability in crypto-
hostile jurisdictions), the _wrong_ way to do it is by adding
#ifdefs in (say) proxy.c. This would require separate compilation
of proxy.c for PuTTY and PuTTYtel, which means that the entire
Makefile-generation architecture (see section D.11) would have to
be significantly redesigned. Unless you are prepared to do that
redesign yourself, _and_ guarantee that it will still port to any
future platforms we might decide to run on, you should not attempt
this!
The _right_ way to introduce a feature like this is to put the
new code in a separate source file, and (if necessary) introduce
a second new source file defining the same set of functions, but
defining them as stubs which don't provide the feature. Then the
module whose behaviour needs to vary (proxy.c in this example) can
call the functions defined in these two modules, and it will either
provide the new feature or not provide it according to which of your
new modules it is linked with.
Of course, object files are never shared _between_ platforms; so it
is allowable to use #ifdef to select between platforms. This happens
in puttyps.h (choosing which of the platform-specific include files
to use), and also in misc.c (the Windows-specific `Minefield' memory
diagnostic system). It should be used sparingly, though, if at all.
D.14 Do as we say, not as we do
The current PuTTY code probably does not conform strictly to _all_
of the principles listed above. There may be the occasional SSH-
specific piece of code in what should be a backend-independent
module, or the occasional dependence on a non-standard X library
function under Unix.
This should not be taken as a licence to go ahead and violate the
rules. Where we violate them ourselves, we're not happy about it,
and we would welcome patches that fix any existing problems. Please
try to help us make our code better, not worse!
Appendix E: PuTTY download keys and signatures
----------------------------------------------
We create PGP signatures for all the PuTTY files distributed from
our web site, so that users can be confident that the files have not
been tampered with. Here we identify our public keys, and explain
our signature policy so you can have an accurate idea of what each
signature guarantees. This description is provided as both a web
page on the PuTTY site, and an appendix in the PuTTY manual.
As of release 0.58, all of the PuTTY executables contain fingerprint
material (usually accessed via the `-pgpfp' command-line option),
such that if you have an executable you trust, you can use it to
establish a trust path, for instance to a newer version downloaded
from the Internet.
(Note that none of the keys, signatures, etc mentioned here have
anything to do with keys used with SSH - they are purely for
verifying the origin of files distributed by the PuTTY team.)
E.1 Public keys
We supply two complete sets of keys. We supply a set of RSA keys,
compatible with both GnuPG and PGP2, and also a set of DSA keys
compatible with GnuPG.
In each format, we have three keys:
- A Development Snapshots key, used to sign the nightly builds.
- A Releases key, used to sign actual releases.
- A Master Key. The Master Key is used to sign the other two keys,
and they sign it in return.
Therefore, we have six public keys in total:
- RSA: Master Key, Release key, Snapshot key
Master Key: 1024-bit; fingerprint:
8F 15 97 DA 25 30 AB 0D 88 D1 92 54 11 CF 0C 4C
- DSA: Master Key, Release key, Snapshot key
Master Key: 1024-bit; fingerprint:
313C 3E76 4B74 C2C5 F2AE 83A8 4F5E 6DF5 6A93 B34E
E.2 Security details
The various keys have various different security levels. This
section explains what those security levels are, and how far you can
expect to trust each key.
E.2.1 The Development Snapshots keys
These keys are stored _without passphrases_. This is necessary,
because the snapshots are generated every night without human
intervention, so nobody would be able to type a passphrase.
The actual snapshots are built on a team member's home Windows box.
The keys themselves are stored on an independently run Unix box
(the same one that hosts our Subversion repository). After being
built, the binaries are uploaded to this Unix box and then signed
automatically.
Therefore, a signature from one of the Development Snapshots keys
_DOES_ protect you against:
- People tampering with the PuTTY binaries between the PuTTY web
site and you.
But it _DOES NOT_ protect you against:
- People tampering with the binaries before they are uploaded to
the independent Unix box.
- The sysadmin of the independent Unix box using his root
privilege to steal the private keys and abuse them, or tampering
with the binaries before they are signed.
- Somebody getting root on the Unix box.
Of course, we don't believe any of those things is very likely. We
know our sysadmin personally and trust him (both to be competent
and to be non-malicious), and we take all reasonable precautions to
guard the build machine. But when you see a signature, you should
always be certain of precisely what it guarantees and precisely what
it does not.
E.2.2 The Releases keys
The Release keys have passphrases and we can be more careful about
how we use them.
The Release keys are kept safe on the developers' own local
machines, and only used to sign releases that have been built by
hand. A signature from a Release key protects you from almost any
plausible attack.
(Some of the developers' machines have cable modem connections and
might in theory be crackable, but of course the private keys are
still encrypted, so the crack would have to go unnoticed for long
enough to steal a passphrase.)
E.2.3 The Master Keys
The Master Keys sign almost nothing. Their purpose is to bind the
other keys together and certify that they are all owned by the same
people and part of the same integrated setup. The only signatures
produced by the Master Keys, _ever_, should be the signatures on the
other keys.
We intend to arrange for the Master Keys to sign each other, to
certify that the DSA keys and RSA keys are part of the same setup.
We have not yet got round to this at the time of writing.
We have collected a few third-party signatures on the Master Keys,
in order to increase the chances that you can find a suitable trust
path to them. We intend to collect more. (Note that the keys on the
keyservers appear to have also collected some signatures from people
who haven't performed any verification of the Master Keys.)
We have uploaded our various keys to public keyservers, so that even
if you don't know any of the people who have signed our keys, you
can still be reasonably confident that an attacker would find it
hard to substitute fake keys on all the public keyservers at once.
[PuTTY release 0.58]